An M4 variant is like the Honda Civic of the AEG world. You can stick with it as is...add external do-dads as needed. You can build it out longer for more of a DMR thing...you can shorten it up for tight CQB. Mags, batteries and everything else is readily available...
The M14 is a great rifle...but long (I'm 5'6" and I'd be hard pressed to "run-n-gun" with that).
The KA FAL series is just plain awesome. The full length is loooong (99cm). The Carbine length is just right for me. The Tactical Carbine is great for CQB. The Carbine is a big gun (matches the "battle rifle" feel)...and it shoots like sex right out of the box.
Because of the ranges involved with airsoft...being up front on the assault sees a lot more action than hanging back. If you're young and if your knees are still good...get up front.