Hey, olsen. Welcome to the board. I am assuming you are american for this response. As far as your choice for rifle, as long as you get some trigger time I think you'll be fine. I think alot of us would reccomend the remington 700p chambered for .308, but it seems like as its a christmas present beggers cant be choosers. Think long and hard before you make the decision to enlist. Once you decide you wish to sign your life away you can do so at the age of 17 with your parents consent. Also I've gotta tell you that your not going to go to basic training, then straight to sniper school. Not unless your screwing the presidents daughter at least (which wouldnt be a bad thing in my opinion). If you wish to join the SEALS then youve got to be navy or marine corps first. The marine corps basic training is 13 weeks long at paris island and its pure fucking hell. If you cant run very well, then dont join. I belive the army basic training is 9 weeks long. in both branches you'll have to be slotted for infantry and pass as an expert marksman to even get an opening for sniper school. As far as the terrain you be working on, they all pretty much work through the same conditions which for right now is sand, sand, and more sand. The seals just do it in smaller groups. All SF units do. I wouldnt discount the army though, as the army ranger's Delta force are just as highly trained (some would argue more so and i think i would agree) than the seals.
Whatever you do, good luck.
Whatever you do, good luck.