Scope
I have heard many good things about SuperSniper, but you should be the best judge since you already own one. For rings and bases, I recommend not going cheap. I have Badger Ordnance 0-Cant base on mine, and Badger Ordnance Medium rings, which bring the scope as low as possible. The key is to get the scope as low as you can without touching the barrel. Badger Ordnance is THE best brand, it costs a decent amount, but its built tough and everyone loves it, you won't find any negative reviews. Our military guys swear by it, and so does everyone else.
The difference between long action and short action is as it sounds. The action is longer on some Rem 700s than on others. This is due to round size. In the larger magnum calibers, the action is longer because the rounds are longer (at least this is my understanding). So, the screw holes for bases are in different spots, as the receiver is longer. You have a short action, you need to buy a base for short action, otherwise nothing will line up, the screw holes won't and also if you have the integrated recoil lug that won't fit right either.
My Remington 700 LTR .308, Badger Ordnance base, Badger Ordnance rings, Badger bolt knob, Harris bipod, and IOR Valdada 4-14x50 side-focus illuminated scope.