If you look to the main menu on the left,you'll find detailed, very easy to follow instructions on assembling a lower.
The least expensive lowers seem to be Eagle Arms from Quantico Arms, $100 plus shipping and FFL, and the Stingers, which are going for $89 shipped plus FFL. If you can get them in person, you save the transfer fee. I've got 4 Eagles, they're as good as any I've seen. Everyone who has the Stingers loves 'em, so I guess it's a toss up. RRA is good too, especially at the price you mentioned.
If you search the AR Board under "build", I'll bet you'll find tons of stuff on what/where to get your parts. My Model 1 kit was fine; only the charging handle didn't pass muster...it felt like it was made from pot metal...other than that, it went together fine, and the fit and finish were okay.
A lot of people like the J&T, and I think the M&As are good too. LOTS of stuff to choose from, so have fun!