Well, you can easily just slap a low magnification scope on your M14, upgrade the hop up and barrel and you generally should be good for a 'DMR' style setup. The AEG will not achieve the same accuracy as a bolt action would, but its really debatable at the ranges you operate in an airsoft game.
Just buy a quality M14 and start from there.
The sniper role in airsoft is really, really fantasized by a lot of people. Its not as exciting as some people might deem it, it really depends on the ability and mindset of the individual to take up that role 'effectively'.