All three weapons can play the longish range support roll you have defined for yourself.
Is it possible for you to try out/hold all of them before you buy? Thats the best way to tell what you like.
Personally, I would go with the M4 if you want a project gun, or the AUG if you want a gun you dont have to do any work on.
It's better to light a candle, than to curse the darkness