A M4 variant is a great choice to start. Now you're going to ask why.
The AEG itself is only one part of the puzzle. When you get the most common gun on the ground you'll be able to find mags, pouches to carry them, furniture to hang off the outside of it, and lots of upgrades for inside.
Get something a bit more off the path and you're stuck with no magazines, or shoving them into your pockets because there are no easy to find pouches of vests to carry them in.
A clear lower receiver one is a cheap buy in. Since mags, pouches, sights, etc. will all often swap between all brands if you decide you need more muscle down the road all the nice stuff you've added on will transfer right over so you won't need to get another whole load of parts.
There are a few others AEGs like the MP5 variants that also boast a big footprint like the M4.
If you're still not sure take the time to visit a few local games and see the gear firsthand.
Airsoft, where nothing is hurt but feelings.