freedom-fighter,
Its real easy in picking a receiver. You have USA or Import. If you use USA then you will use one less USA part when you build the rifle. As for type, you have two choses, full or light'g cut(less weight).
Now the big question, If you post a location, then maybe someone locally would help you build the rifle.
Receiver is going to run you around $200.
Parts kit are from $120. to $250. If you go with a cheap kit, Add $100. for a new barrel.
USA parts and odds/ends are going to be $100.
I have built using DSA's, Imbel's,and Enterprise's. To be truthfull, when It was all said and done, they all shot under 2MOA. But, you have to start with a clean barrel and a fresh bolt or you will end up with trash.
If your looking to just buy a built rifle, then your best bet is to buy from DSA. They will have a new barrel, and fresh parts. Some parts may be take-offs, but if the Take-offs are bad, they go in the trash can or sold to others, but not in one of their rifles.
If you are planning to buy a built rifle for $500. then you will spend more in the long run just replacing worn out parts that never should had been used in the first place!!!
Go to WWW.fnfal.com for more details on parts and builds.