A 30-06 with a good deer bullet would more that suffice. A 190 grain Hornady cup and core would work on plains game, and you will need to bait the leopard, and it will work on the leopard. A 50 yard shot on a leopard is a long one. A 300 yard shot on plains game is a long one. You won't need to install a muzzle break or need a 50 power scope. A good 3x9 or 3x10 scope with a 40-42 mm objective will work well. Just make sure it is dependable and bright.

I used a Leupold 6x42 fixed power on mine and had no complaints. More power is nice for punching paper, but I never had any problems on game ranging from jackals to giraffe with the light, bright, and tough 6x42 Leupold. The 4.5 inches of eye relief is nice as I don't have to lean into the scope to get a good sight picture. Of course, it is your decision to make. Just my .02$.