I'll be doing base <-> mobile, base <-> handhelds, and handhelds to handhelds. Simplex. Analog. Transport company.
Should I be requesting between 150-174 MHz or above 450 MHz? (for my information)
I'm not trying to be flippant here, but what is your real goal here, have your own radio system or have a way for your transport company to communicate (yes, this is a serious question). If you simply want your own radio system then apply away, spend your money on radios, licenses, installation, etc. and have fun. If your goal is to communicate a build-it-yourself system will be expensive, troublesome, and probably not going to provide you the results you're looking for. There are several reasons for this.
First off, a VHF-Hi band frequency will be pretty difficult to find. They're basically all used up (well, taken is probably a better word since they may not actually be in use, but that won't matter for someone trying to get a new frequency on that band). The UHF band is also pretty crowded, but your chance of getting one is better. You may be lucky, but using a frequency coordinator (as suggested above) will be the only way you will find a good frequency.
Your 20 mile range Base <--> Handheld will require a nice, tall tower (or a nice, high remote base site), quality antenna, and low loss commercial coax. None of these will be cheap. The Base <--> Mobile 20 mile range will be easier to accomplish, but still will require a pretty good cash outlay to do so.
Using a radio company's existing infrastructure will buy you two things. First, it's up to them to build and maintain the system. The coverage is "guaranteed" (note the quotes here, it's because you'll never get 100% coverage) and you should be able to discover where you will and where you won't have coverage prior to spending much cash. If you build it yourself, you'll have spent your wad and may find that important areas provide only minimal coverage at best.
Also with your own system, you'll need to spend lots up front and spend some more as you go for maintenance, repair, and unless you're really lucky and have a great site available at no cost from your "Uncle Bill" the rent on your tower or roof-top space won't be cheap. Going with a radio company, you may be able to rent the radios along with the air time so your build cost may be rather small.
Doing it alone, you'll have a single site system. Where it reaches, it reaches. Where it fails, it fails. Your expansion options are very limited as well. It may not be possible to upgrade it from simplex to a repeater type system (there may not be an available frequency when you decide to upgrade) or you may be forced to change bands to do so. With a radio company's system you could be one user on a large trunking system that may have several sites available for you to use. Other sites may be linked in to allow you to expand your coverage by simply purchasing additional coverage from them, probably just needing your radios be reprogrammed, or perhaps without changing your radios at all.