Go for a decent brand -- Rock River Arms, Stag Arms, DPMS, Colt, Bushmaster, etc. There are stories about Olympic Arms and a few other brands where those parts don't mate well with parts from other manufacturers. (There are also higher-end brands, but they usually are hand-fitted together and come as complete rifles.)
What you will need depends on where you live and what the local laws are. ARs are illegal in any form in some parts of the country. An AR lower is the rifle in the eyes of the Feds, so they will do a NICS check when you buy your lower. Basically, you fill out a form and they call it in. It takes a few minutes. Assuming you are not a felon, you should get approved. You will need a picture ID (like a license) for the NICS check.
As far as pricing goes, check ar15.com in the Equipment Exchange and look at what new lowers are going for. Some will be complete with the lower parts kit (trigger group, grip) and some will be stripped. Price out complete lowers unless you enjoy fiddling with assembling stuff.
Good luck with the gun show. The ones around here don't bring many deals. Here's to hoping you have better luck.