The 20 gauge shot gun is probably a better option in my opinion for hunting white tail deer with slugs.

The 12 gauge slug weighs more and does not have a significantly larger amount of powder behind it.

In my opinion, I would purchase several boxes of slugs and take the gun to a range and shoot it at the range you expect to shoot it at - with the same clothes on that you would be wearing when you were hunting.

Find out which shells were the most accurate and go back to the store where you bought the original shells and buy several boxes of the identical shell - same lot number etc..

Neither the shotgun nor the scope is going to be a real tack driver at 100 yards, but you might get it sighted in good enough to keep it in the center of the target ( 3 inch ring) 3 times at 50 yards.

I started my hunting career with a model 94 Winchester 30/30 rifle and it was not much better then a 20 gauge shotgun with slugs.
I don't remember anybody ever getting anything with that rifle.