Zou,
I'm going to try and answer all of your questions the best I can and going to apologize in advance for the length.
North Central trucks to my knowledge actually transmit on 154.400 but dispatch callsback on 154.325. You probably don't recall but StL City was this way. Basically if you were sitting in North Centrals dispatch center, you would hear the trucks on 154.400, but would answer them on 154.325. Again I dk if this is accurate as I can't hear trucks as far as I am anyway. However I occasionally hear dispatch answering on 154.400. Best bet Zou is program both.
IMO in forum talk is "In My Opinion"
NMO is a type of antenna mount it stands for "New Motorola Mount". If you were to unscrew an antenna on 99% of the trucks around here, it would be on this type of mount. Now this can be permanently installed on a roof of a vehicle or on a magnetic, trunk, mirror, gutter mount.
I have a NMO magnetic mount with coax that was already attached and then I crimped on a BNC (what connects to a scanner). If you're not good at crimping and stripping wire appropriately then just search online or ebay for a NMO mag mount and make sure it has a female BNC on the end. THEN search for a NMO VHF antenna. Make sure it list the frequency range and at least covers the 146-174 mhz area. Some vhf antennas for sale are trimmed to transmit on the ham bands of 140s so your coverage on the FD frequencies will suffer. I have an Antenex B1443 VHF 5/8 wave 3db gain antenna screwed onto a NMO mag mount. I haven't trimmed it either. I have a metal bracing outside my upstairs window that it's holding onto. Then I just ran the cable through my window I keep slightly cracked. It works until I get a more permanent install.
I can't tell you what will be best for your situation because there are many many factors involved with antenna reception. I've bought many many antennas throughout my years, most used and on ebay. You could always buy one mount and a few antennas until you've found one that works. I've never transmitted, only received with them. I have found that some I had, worked better when I lived in the country, that all of sudden were junk in suburbs. Without getting too technical if you have an antenna that is "unity gain" then it's signal propagation (and receiving characteristics) are fairly uniform throughout the horizon. Basically your antenna will pick up signals in equal (or close to equal distance) in all directions. If you have a higher gain antenna (7-9db) such as most "yagi" types, the signal is more narrow like a cone but you gain distance. So for you it may mean that if your antenna will be on roof then probably a 0-3db gain would be sufficient. I'm reluctant to suggest a yagi as they are quite expensive and even though your reception will be better in the direction it's pointing, you will sacrifice in other directions. Here's a link that explains it a bit better.
Antenna Gain Explained : Will a high gain antenna actually suit your purpose ?
I'm by no means an antenna expert, only can offer experience in what i've used. Try asking questions regarding antenna setup on the antenna forum, there's some really sharp guys over there. Remember that everyone is going to ask and suggest you get your antenna outside and higher. If you're in a basement and using a back of set antenna, you're very very limited, to the point of screwed trying to monitor anything other than North Central. If whoever owns the house doesn't mind, look around your basement for an outlet for cable tv. Trace the cable and see where it goes. It may go up to roof to a satellite or antenna for tv. If it's not in use find where it terminates and we can help you from there on how to hook up antenna. Even if the antenna can be placed in attic you will pick up a lot more. If they'll let you I'd say mount an antenna outside on roof and drill hole and run cable to basement that would do wonders for you. Again outdoors and height are your friends, then worry about what type of antenna. Also a high gain antenna in basement will not help as it's going to be blocked by not only house but trees and such. Antenna theory will give you a headache after awhile but search the internet and learn before spending boat loads of money. I know you're a medic student, i've been there, as tempting as it may be, save your money cause things change so quick with radios around here. Not to bum you out even more but North central will be moving to Central county so no more dispatch center up north. But for now try placing you scanner and/or antenna with orientation to St. John since that's where they are located. They have a feed on here as well.
To answer your 800 mhz question I'm going to try and be as polite regarding the situation and non-political. I however am a very honest person so here it goes.
The new 800 mhz system does have some factors that are an improvement over older systems.
The fact that everyone is going to it is the #1 benefit, in that interop problems SHOULD be a thing of the past (although doubtful). Your communication distance is fantastic. On certain talkgroups that use statewide towers, you can talk all over country. We tested it and were communicating with a tech in KY from here in IL. If the system is installed properly, you should never have any issue with anyone including portables to comm centers in hearing you crystal clear. At 700mhz (where you will transmit, i'll get to it later) you have better chance on getting signal out of buildings and through obstacles but you lose power. Therefore repeaters and amplifiers need to be placed strategically in buildings, in trucks etc. The new system is digital so it's supposed to sound better but I disagree. Another advantage to some is it makes it harder for the public to monitor.
Now the problems with digital 800
It's expensive. It uses more computers. When you key up a radio you are transmitting in the 700mhz range. The signal hits closest tower, goes to computer, is assigned a frequency and is retransmitted on the talkgroup on 800mhz. So if computers are down, no system. Everyone will argue that this is uncommon but I wouldn't rule it out. Also if you are in a building or somewhere and key up and don't hit a tower then no one will hear you. It's complicated to program and motorola doesn't allow depts to program themselves anymore with this system. I could program a Vhf radio in 10 minutes but have no clue when it comes to the new radios and wouldn't be able to if I could cause motorola is strict with it. Part of this is motorola not wanting everyone to be able to program the radios and making you spend more money (it's the truth).
Now why has everyone gone to P25 digital TRS 800Mhz. Motorola was losing money in the VHF/UHF world. I've been in this field for years and VHF around here was the single most used bandwidth for public safety. Radios could be purchased used, programming cables and software had over internet for cheap. Therefore nobody really had to use motorola for anything anymore. Not to mention companies like Relm, vertex, Icom etc that sold vhf and uhf radios for dirt cheap and offered programming software for free at times. My volunteer department at one time had 3 different manufacturers of radios and never had any issues. Trunking itself is not new. StL PD has been trunked for a long time and FD was once trunked for short time but they didn't like it.
Now here comes my biased opinion and I know some who may read this will argue but it is what it is. Motorola needed a way to have control over the programming and sale of radios and maintain a monopoly over two-radios and public safety. They developed the P25 or Project 25 system. The radios have GPS, over air programming, encryption blah blah blah. Motorola sells it as a new improved way of communicating, hitting people with how it'll enhance interop, clarity, scanner heads can't monitor, again blah blah. Their angle, now they have full control over every radio they sell. They can track and kill radios that are not "legal" just like a cell phone, in fact they are basically a nextel. Sure you can buy a XTS portable that'll get on systems but once the admin sees an unauthorized radio, bye bye, now you have an expensive paperweight.
Now I will say that the old Vhf had problems. Distance at times was limited especially rural, some installed repeaters but they come with issues too. Like I said earlier anyone could buy a radio and program it. I remember one night at my old FD where we listened to a guy preach about the end of the world on our dispatch frequency. The problem always seemed to be interop and certain people not having appropriate frequencies to communicate. It'll be interesting to see if the new radios will even have interop channels. Were apparently getting dual band radios that can transmit on both VHF and 800 mhz but I'm skeptical on how this will be used. To answer your question about the station alerting. At my FD for example we have our lights and what not hooked up to a plectron that monitors VHF not 800. There's also no minitor pager that does 800. But guess who has a station alerting system they are pushing lately.......Motorola
As a FF who has used the new system and will be again soon I'm excited but nervous.
As a scanner nut I'm furious. I become more and more upset when I hear of a state going to this system some of whom are going encrypted. What's funny is hearing about cities and states that went to this system or similar and went back to old cause it had bugs, but not a common thing. And I applaud those who basically tell motorola to shove it and they stay with what's been working for them.
Hope this helps clarify a few things. Remember to use the search and look on the missouri forums cause a lot of this digital trunking system talk is on there as well.