I would build it with PSA lower build kit, enhance to the Magpul kit I would recommend, and then top it with a mid length complete upper. PSA was running some sales on their middys (and may still be)-- the 2 I got have been awesome. You could get a premium upper with the BCG and CH pretty easily for 600 bucks(ish) delivered, and you would not have 725 in the whole thing. Grab some Magpul 30 round mags from one of the half dozen places with free shipping, at less than 15 bucks per mag, throw in a rear sight (like an MBUS) and run the snot outta it!