The first thing is to be realistic about the effective range. Shotgun slugs lore on the internet is insane. I grew up in a shotgun slug deer area, have seen hundreds of slug guns, have shot tons of them, own half a dozen. I don't care about ruffling feathers anymore, because anything else is only doing a disservice to others. Shotguns are NOT MOA rifles. They are NOT 2 MOA rifles. There used to be a fun little online club called the American Slug Shooting Association, or ASSA. It was basically a group of guys who loved slug guns, and put together informal competition to see what they could do. In the purpose built rifled slug guns, guys would often shoot 2" 5 shot groups at 100 yards, but not every time. You got unlimited attempts per month, so it wasn't like this happened regularly.
What I'm trying to say is if you think you can slap a rifled barrel on a pump action shotgun and shoot 2" or better at 100 yards consistently you are going to be wasting your time. A lot of that more accurate ammo like the Hastings and Lightfield slugs are no longer made. Neither are Buckhammers. A lot of the sabot slugs made today are only made to do one thing, shoot fast, and sell faster. There are good ones, Remington Accutips are often suggested. Federal powershok sabot slugs are another good one if you find them. Realistically you are looking at a gun capable of consistent 4" groups at 100 yards with the right ammo. You may or may not feel good for a 125 yard shot after trying it. You can probably put the idea of taking a 150 yard shot out of your head. 150 yards is asking a lot from even a purpose built slug gun like a Savage 212 or 220. Plenty of people have shot deer at 150-200 yards with slug guns, but I'm sure not taking a shot at an animal when my groups could be 6"-8", that's leaving way too much to chance. It could be even worse. I like to cast my own slugs and shoot at 200 and 300 for fun. At 200 yards sometimes slugs go haywire. Winchester BRI for example is one of the most accurate you will find at 100 yards, but seem to tumble sometime past that.
For sure go with a 1-4x. Nothing wrong with a red dot either. Drop is almost not a consideration, so no need for any reticle. Your accuracy will run out way before drop is a factor. Even a low recoil 1250 fps slug is going to be plus or minus 3" past 110 yards. Most are more like 1500-1700 fps.