It sounds to me like deer hunting is your primary concern, with turkey hunting or anything else a remote possibility.
That being the case,
since you said you want a rifled barrel and would scope it, the Mossberg Combo is NOT the route to go.
If you scope the Mossberg, then you either need to drill & tap the receiver or go the B-Square route, neither of which is a good choice if you ultimately want to use the bird barrel. There's no such thing as a cheap sabot or an inexpensive day at the range. Changing barrels would force you to re-sight the scope each time.
I don't know if the Mossberg combo is available with a cantilever barrel. That would be a much better option.
Remington 870's are available with cantilever barrels and a bird barrel could always be bought at a later date.
I'm sure you could find an abundant amount of Remington barrels on Ebay for a fraction of the cost of a new one.
Do yourself a favor and pick up a Mossberg and then pick up a Remington.
You'll feel the difference in quality of the Remington for a relatively few $$ more.
My suggestion would be to purchase an H&R Ultra Slug Hunter in either the 12 or 20 gauge, both of which can be had for under $200.
http://www.hr1871.com/Firearms/Shotguns/ultraSlug.aspxMake this your dedicated deer shotgun. No better shooting slug gun for the money.
If you find you want to take up turkey hunting or other game, buy yourself an inexpensive shotgun at a later date.
There's plenty to choose from.
Oh yeah, forget about what you read about 200 yd sabots. They exist but they don't tell you that the recoil will rattle your brain.
They'll fly that far, they can kill that far, but the reality of you or anyone else
consistently hitting where you aim is remote.
Realistic slug hunting is inside 125 yds. The other 75 yds. is marketing!