Starting off with a sniper rifle pretty much requires you to have patience practicing when you start-which means you won't be able to get a lot of out of Airsoft right away. You'd need to sink a lot of cash into your rifle to get it up to specs for milsim. And sniping, as I've heard, is nothing like sniping in real life (which I've done a few times).
If you'd like you can try DMR, in which case the G&G GR16 R5 is a great rifle. Long barrel, RIS for sights and whatnot, and it still shoots auto in case you need to. You will need to get a scope riser though, the front sight gets in the way of your scope if you just mount it onto the rails as is.