You can load them with a .308 bullet, but the barrel groove diameters typically run .312-.314. You might be better off using a bullet for the .303 brit which is .311" in diameter, so long as you slug the barrel and make sure the barrel is at least .311 in the grooves.
I figure that if it's hunting accurate (paper plate at 100 yds) and won't blow up in my face, it's probably a good deal.
Given that the bore isn't rotted out it will probably shoot paper plate accuray at 100 yards, but not much more. It won't blow up- the mosin action is a very strong action. A lot of the mosins up for sale right now are in better shape than the ones available 2 or 3 years ago. Most on the market have been rearsenalled or even 'new in arsenal wrap' condition.
Surplus ammo is dirt cheap. Good Czech surplus ammo costs about $35 for 440 rounds, I shoot mainly albanian surplus because I can buy it locally for $50 per 340 rds. All of the surplus ammo is corrosive.