Like they have said, any of the 7mm mags will work perfectly for what you intend. The RUM is the biggest and has the most recoil, but it will certainly get the job done.

If you really want to get proficient with one gun, I personally would select something that kicks a little less. I' d go with the 7mm Rem Mag because I know I can shoot it off a bench without discomfort and the ammo is quite a bit cheaper than the other 7mm mags.

Regardless of what you get, 160gr bullets should be all you would ever need.