It depends on what you want to do with it, your eyes and your budget. Also the preferences of your particular gun.
At ranges up to 50 yds almost any rimfire scope will work well. Even a centre fire scope with fixed 100 yds parallax setting works unless your accuracy standards are very exacting. The Bushnell rimfire and Banner series of scopes are good and cost between $50 and $125. There are many other adequate scopes in this price range. Variable power scopes (i.e. 3x-9x) will handle both target and hunting uses.
For accurate 100 yd target shooting you will probably need better glass. If your eyes are very good you may be able to see rimfire holes clearly at that distance with a cheap scope, like a Tasco 6-24x varmint, but maybe not. I personally like the Bushnell Elite 4200 6-24x for this purpose. It's bright, clear, and has a lot of mounting room so you can position it where you like. These cost about $585 new, but are often available used for less.
Ammo choice is also dictated by the use and the particular gun. You can fire anything in a Savage MKII, but some people have ejection problems with some brands of ammo. You just have to try various brands and settle on what works and meets your needs.
For hunting and shorter range plinking most of the bulk high velocity rounds are fine. If you want tight groups at 100 yds you are better off with a good quality standard velocity cartridge. For this purpose I get good results with SK Standard + and Eley Sport. Both are in the $6/box range.