I personally would recommend against an entry level AR/M4. If you do get one, the Combat Machines have a good reputation but the only one I've encountered failed so often and consistently that the owner got so fed up that declared they were quitting. Don't know if they followed up on that or not.
But of the AEG's that I've owned my China AK has been a work horse. I've abused it, scratched it, let it rust, dissembled it but put it all back together and it still works great.
If you want to stick to that price point I'd get one of the many varieties of China AK's. Dboy's Ak's have a good following, and people seem to swear by them. Please note I have no direct experience with this manufacturer.
http://www.airsplat.com/Items/ERM-BI-74-RK03.htmIf weight and length is a concern for you you might want to look at the APS UAR.
The Gearbox is the common Version 3 with some modifications. Primarily to allow it to be adopted into a bull-pup shell and a quick spring change feature. An AEG I own has this feature and it makes working on it a dream. But I must point out that the mag well on this tends to be temperamental because of how the Gearbox sits inside of it. The modification is simple to do to allow the use of any mag though.
Again no direct experience with this companies products.
http://www.airsplat.com/Items/ERM-APS-UAR.htmIf you want to stick with the version 2 gearbox, by all means stick to an AR, but the G3 and MP5 both use the same box.
The G3 has a long barrel and highly upgradable internals and an optic can be mounted with the use of a rail adapter plate. The G3 also has a large space for batteries. Again no direct use of this companies products.
http://www.airsplat.com/Itemdesc.asp?ic=ERM-JG-T3K3-100So please look at other models before you choose. The M4 is everywhere, overbought, and over used. Hell I own one, and all I've done tell you to look at other models. But I do like my M4, it feels nice in the hand. However if I really wanted to I can go on a rant about it's ergonomics, layout, and its overall design.
I'm a major chairsofter, sorry about that.