As someone who's assembled several AR variants from scratch, I'll say don't try it unless you:
1) Really know what you're doing;
2) Are willing to put in a lot of time and effort, including having to dremel the crap out of stuff, and;
3) Have really deep pockets, because it's most likely going to cost you more than buying an off-the-shelf AEG that's ready to go: right now you're looking at the receiver and fore end and stock and going "that won't be so bad" but then you'll find yourself missing little bits and pieces here and there, or having one part that isn't compatible with another (see #1) and you have to start hunting down and ordering all that stuff and it adds up.
Yes, you can cut corners. You can fudge stuff. But then you'll just have paid as much as you would have for a good gun, just to end up with a sub-par gun.
I do it because I love doing it. But it eats up a lot of time, money and effort.