SG Has the Privi stuff for $3 cheaper per box.
There is nothing wrong with shooting surplus ammo, even if it was a pristine numbers matching original.
Some people seem to think that "corrosive" primed ammo is caustic and will damage the gun, and this is not true. It is only corrosive because of salts in the priming compound, which will attract water, and therefore lead to rust if the salts are not removed. All this means, is if you shoot corrosive ammo, you just need to clean the gun shortly thereafter.
In reality, every round fired through a military surplus rifle up to the point they ended up in the hands of a collector, was corrosive.
As to the rifle, if you are happy with it, keep it. There in not really anything wrong with a Mitchell rifle, it is just that what the rifle is (A heavily reworked and polished Soviet Capture/rework) does not match what the advertising claims it is (Rare original unmolested collectable rifles.) They are good looking shooters, and if you want a good looking shooter, you are all set.