With regard to which sites to program, a little research may be beneficial. Look at this system, which is for VIPER:
https://www.radioreference.com/apps/db/?sid=7118
When you get to this page, you'll see a list of sites. Examine those that are in your county and in the counties contiguous to your county. Then click on the site name which opens another page. That second page will show the Range for that site and a circle showing the approximate coverage area. If you are in that circle, it would be reasonable to program the site. The only caveat is if you aren't interested in transmissions for that area, there is no benefit to programming it. For example if you are close enough to Raleigh but don't want to hear anything "Raleigh", don't program Raleigh.
I think it's better to put too much in at first just to see what you can get, then delete those you can't.
A couple of other thoughts. The 800 spectrum radio waves won't travel as far as something in the 150 spectrum. Also given how statewide system sites interact with each other, power output is not as high. The way it works is that if a unit from County A travels in to County B, the radio may be allowed to affiliate with the County B site and then the County A and B sites are "linked" together so that unit can still communicate with its "home base".
The "newfangled" statewide trunked radio systems are a definite breed of cat as compared to conventional stand alone systems of old days.
A different antenna could help some, but an outside antenna as high as possible with as good as affordable coax will give the best results possible.