The charger/amplifiers use a 110vac to 12vdc power supply, so feeding it directly off of 12v from your Jeep should not be a problem. You'd need to cut the cord off the power supply and determine which wire is positive and which is negative.
The AC power supply puts out 900ma, so installing a 2 - 3 amp fuse would be fine.
So, what you need to do is:
1. cut the cord off the power supply.
2. probably need to lengthen wiring to reach power source.
3. install fuse on positive wire very close to power source.
4. connect positive to power source and negative to Jeep body.
5. make sure everything is correct before plugging it in.
You can power it right off the battery, they don't draw much when the pager is not in them. If you are up to it, it would likely be a good idea to find an ignition switched power source so it won't draw power when the Jeep is turned off and drain your battery. This will take some investigation to find a source, likely near or in the fuse box.
As for the antenna....
You need to decide what sort of mount you want to use, permanent (recommended), bracket (second choice), or magnetic mount (last choice).
The antenna connector on the back of the amp/charger is a BNC, so that's pretty easy to find.
I'd recommend using the standard NMO mount as it will give you the most options for antennas. NMO's are pretty much the de facto standard for professional mobile antenna mounts. You can purchase antenna mounts with cable and BNC already attached:
NMO Hole Mount with Cable & BNC
Ideally, you should use a permanent mount as it will give you better performance, better looks, and a longer life span. It does require drilling/punching a 3/4 inch diameter hole in the vehicle. That part isn't for everyone, so going with a bracket mount off the fender would work too.
Magnetic mounts are the least preferred options as the tend to damage paint under the mount, and in my own opinion, look unprofessional.
As for the actual antenna, that is going to depend entirely on what frequency your department uses for their pagers. It's going to be UHF, VHF High or VHF Low. You need to determine that before you choose your antenna. You need to have the correct band antenna for best performance. Let us know which one it is and we can assist you in finding the correct antenna.