I say G&P's make the best project guns. Yes they're expensive and yes you might end up replacing most of the part on it. But when your done, you have almost a whole AEG in parts that are actually WORTH MONEY!!
I made back a good chunk of change on my fully converted magpul AEG because I started off with a G&P and sold the metal body just after they stopped being widely available.
ANYWAY, start with a G&P, it's a SOLID platform, VERY rigid gun. That takes most of the inconsistencies out right there.
I recommend a 6.03 for an AEG DMR. Prometheus barrels are very consistent with their bores (that means very accurate), and the bore is open enough that you don't need to clean your ammo pre-game or clean your barrel every 50 rounds.
Remember that FPS doesn't equal range at all, but it helps you get your rounds on target in a shorter timespan.
Get a firefly hop rubber, and high quality compression parts. So prometheus cylinder set, or ARS stuff from Amos.
And find yourself an excellent gundoctor. $300 worth of prometheus parts doesn't mean crap unless the guy putting them in knows what the hell he's doing!