If you have good eyesight and will be hunting in full daylight a $40 "target sports" will work.
If you will be ready to shoot at the "legal start" of shooting time, or are losing your eyesight you will need a high quality scope, either a Burris 2x-7x, or Leupold 2.5x-8x, preferably matte black finnish, but definitely not silver finnish (the Leupold scope is silver on the inside and it'll give you a glare).
Then you will need to learn to keep your dominate eye on the animal while you bring the pistol scope in line with your sight, you do not look through the scope to find the target.
As for durability I haven't had any problems with the glass, but have had a mount break on a red-dot sight, and a scope slip in Leupold QRW rings. My favorite ring/base combination are Weaver #92 base with their 4x4 medium rings, no slipping and the square lugs makes good contact with the base. And if you want a 3 ring set-up the spacing on the base is already correct for these rings.
When you mount your base, check that each screw individually holds the base to the barrel, if it doesn't you'll need to shorten the screw. If you are going to drive that 45-70 hard you should consider having a 6-screw base smithed onto the barrel.