Building a custom M16/M4

So I have decided that I would like to build a custom full-metal AR variant rifle over the next year. I have no idea what parts I want but I know I'm buying a complete gearbox rather than building one. Does anyone have any tips on what to get. I have around a $250 total budget. Also what gearbox should I get and does G&P make good metal recievers?

Dboys also makes good receivers, go to ebaybanned, or ebairsoft.com for those.

I would try to salvage as much as possible from boneyard guns. Don't go for one that only has one of the parts your looking for, but several like a stock and a handguard would be a good buy, possibly with a outerbarrel too.

Ya but the problem is that I plan on picking all the parts out.

exactly why you have to be creative, keep doors open for the "looks" because you want something that looks sharp but doesn't break the bank because your trying to cherry pick each individual part which is way more expensive.