Re: Base and Rings for Savage BTCSS
The model 12 is a short action. If it is a new production action, it will be a round rear action.
The 0 MOA base is flat in relation to the bore. The 20 MOA base is tilted down at the front. It lets the scope adjustment be centered when shooting at extended ranges.
Do you need it? Depends on the range you want to be shooting. The Leupold you listed has 75 MOA of travel. That will get you a long way even with the flat base. The 20 MOA will let you go even further, way beyond the effective range of the 22-250. Since the scope has a decent amount of travel, you should be able the zero at 100 yards with the 20 MOA base.
Rings are a subjective topic. My favorites are Burris Signatures, or Warne Maxima WD's if I need to swap scopes often. Other's have their favorites, but those two have worked well for me.
EGW bases are tough to beat for the price. Also take a look at the new Weaver Tactical bases.