It all depends on how good of a shot you are.
For example, my father of 82 years can take his old Model 12, with a single brass bead and drop a running doe @ 100 yards every time. His longest shot was 175 yards at a running buck. He don’t miss. He shoots Remington 3″ rifled slugs. Don’t ask me how he does it, but he has an unbelieveable eye for shooting.
I, with the same gun, struggle at 100 yards and I do consider myself an “above average” shooter with records to back it up.
I shoot an 870 Wingmaster with a Hastings Cantilever scope mount, 4x Simmons.
If you are going to restrict yourself to only shooting smooth bore, then focus your attention on Brenneke, Remington, Federal, Winchester slugs.
If you scope mount, I would zero at 50 yards. Your longest daring shot for the average shooter with rifled slugs shouldn’t be much more than 75 yards, with 100 yards pushing the limits. Again, for the average shooter.
My opinion is that since you will be limited on the rifled slugs, I would be a little hesitant dropping $100 to $150 in a scope and mount. Try shooting the slugs with the beads on the barrel and see how you do!!!
Especially if you are hunting woodland, most of your shots in the woods won’t be over 30 to 50 yards. You really don’t need a scope for it. If you are hunting field edges, then it can be a different story. Here, you may be left watching deer (even a super buck) walk away @ 150 yards or 200 yards because they are “out of range”. If you are hunting open land, then go with the scope and focus @ zeroing at 75 yards, if you are a good shot, focus @ 100 yards.
The old saying is that you should be able to hit a slug box @ 100 yards with a scope at the range.
As for which slug? In rifled slugs, each barrel performs a bit differently. Try the 4 manufactures I mentioned above in different slug sizes and see how they perform in your gun. I would probably rank Brenneke as one of the best in accuracy and distance performance over the rest.