First of all, I recommend checking out
www.ar15.com they tend to be very knowledgable on the subject. Secondly, if you want to buy an AR15, but you're not sure which one, there is another alternative. The only part of the AR15 that counts as the firearms is the stripped lower receiver. The rest of the parts can be shipped straight to your house. You can do anFFL transfer on the lower, and then assemble the AR whenever you feel like it. Even if the Brady law is repassed it doesn't matter, since the law grandfathers in AR15s and you technically own the gun when you own the lower. If you want a cheap lower, check out
www.eaglefirearms.net/or
http://www.ar15.com/forums/forum.html?b=7&f=21. It is generally cheaper to build the AR yourself, and is very easy to put together (once again, ar15.com has very good step by step walkthroughs on building your own gun). For example, to assemble an upper receiver to a lower receiver takes about 5 seconds (all to have to do is snap two pins in place) I built my own AR out of 100% Bushmaster parts and saved about $200 from the retail price of the same gun on Bushmaster's website. Like you said earlier, I wouldn't trust people at a gunshow. They can use a reputable name on the lower and assemble the rest from the cheapest parts out there. Only buy from well known dealers. If you have more questions, just IM me.