BP has pretty much said it all.

TheBelly touched on a very good point as well. If you want a rifle that is going to run properly, the two parts I wouldn't get cheap with are the barrel and the bolt carrier assembly.

And, as BP mentioned, if you don't already have some of the special tooling necessary to put the thing together, you can probably expect your cost to be well over $1000 even for a "budget" build.

Definitely not trying to discourage you. It's a fun hobby and you can have a rifle set up just the way you want when you get it done. Just trying to give you a dose of reality to make sure you know what you're jumping into.