I've built a couple of AEGs so far...a couple from scratch and simply modding exisiting ones.
If it's your first AEG....buy something solid and go from there. You might not end up with much of anything from what you started with....but you'll have a higher chance of success along the way. You'll likely end up with 90% of the parts that you need to build a second M4 (or rebuild the original one depending on how you look at it).
If you're familiar with AEG parts, manufacturers and options...then scratch build if that'll get you what you want.
You will get a nicer build doing it from scratch if you buy good parts.
Do not scratch build with clone parts...stick to G&P/KA. If you picked all G&P parts they'd likely go together like butter and look sweet. If you picked a bunch of clone parts from different makes you might very well end up with a crooked stick with 8 shades of black (I kid you not...this is entirely possible).
I just built 2 gas rifles this weekend. Actually better to say that I rebuilt them. One was working...ok/so-so...the other wasn't assembled. I worked back and forth between the parts and figuring it out as I went (they were my very first attempts at building up magna system rifles). Even after building/rebuilding/repairing several AEGs, I'm glad for these gas rifles I bought something that was sort or pre-built already to get a running start at it. I've received literally baggies of parts before and been asked to assemble a rifle out of it...it's not hard, but it's not the nicer way to go.
** if none of the above made any sense to you....don't scratch build...buy an AEG and mod it piece by piece **