Well, let me make it easier for you to decide... What is your maximum budet your willing to work with ?
#1) I ask becuase, sabot slugs (used in rifled barrel 12-ga shotguns) cost close to 2-3 dollars per shot (not per box)... Usually $12-15 for 5 shots. This is the worst ammo to use for "practise", they are TOOOO costly, even if you reload, the reloading components for them will cost you about 1.5 per shot. you will not get much practise with these sabot slugs and at 100 yard shots so you'll never have room to improve your accuracy.
#2) If your not going to use sabot slugs, but instead ur going to use "regular" ko slugs or rifled slugs or what (the once you can use in a smoothbore barrel) your not going to get much accuracy at 100 yards. Some peeps get like 7" groups which is pretty sad for hunting. Of course I have NEVER tried rifled slugs from a smoothbore, but peeps say they are not accurate at all @ 100 yards. Plus they cost about 50-60 cents a pop.
A smarter choice for you is to get a nice pump or semi auto shotgun (with smoothbore only) and use it for your bird hunting. and get a semi auto or a bolt action 30-06 for your deer / black bear. That way you can have both worlds.
You can get a nice used pump gun (I strongly recommend remington 870 wingmaster with 26" 3", you can get one from $200-230). And you can get a nice semi-auto 30-06 (used), something like a browning BAR (and this is one of the best semi autos in the industry for hunting class) for around 400-450. Or if you don't like the semi 30-06, get a nice bolt action 30-06 (like a Howa 1500 or Mosberg ATR) Howa is like 300-350 and the Mosberg is about 250-300. Both are GREAT guns.
Now, spend an additional $125 on a Nikon Prostaff 3-9X40 from
www.theopticzone.com (life time warrenty, can't beat that, anything that ever happens to the scope, they will repair it or replace for you for free).
One more option you have is to get a Spartan spr 453 semi-auto 12 ga in 26 or 28".... Those are the best bang for the buck, $350 brand new semi auto smooth bore, has GREAT reviews.