Having just recently gone thru a complete AS gun rebuild, including most of the pieces of the mechbox, of a basic AK to something very customized, I can testify to the truth of Tokyo 7's suggestion.
I am a business systems tech, and no slouch in the 'handyman' dept. Learn to be a airsoft tech with a working model to refer to. It is SO much harder to diagnose problems, from too many parts added all at once, as opposed to getting you gun complete, observing its normal operations, and customizing one piece at a time (if needed).
I especially like her suggestion of buying the mechbox complete (some very nice reinforced, pre-shimmed models avail) and then parting everything together after that. That suggestion alone, would make things sooooooo much less painful.