Once you figure out which rifle you're going to purchase, here are a few tips to get what you're needing out of it...
Each rifle has a "sweet spot" that will produce the best accuracy and effectively burn all the powder INSIDE the barrel (ie: no flamethrowers!). My Encore with a 26" barrel shoots a 240-260 grain saboted bullet best with about 75-80 grain of Triple Se7en. It's a very manageable recoil and extremely accurate out of my rifle (.5 MOA out to 200 yards!). You can get by with a little less powder if you're recoil sensitive and you should still be more than accurate enough for deer hunting.
You can go with a lighter weight bullet for less recoil and a little more speed to make up for less powder, but as was mentioned, pay attention to the caliber of the bullet in the sabot as you may dip down into too small of a caliber for your state rules when you lower bullet weight. You will have the greatest variety of saboted bullets (both weight and style) going with the 50 caliber. I have a 45 caliber that I hunt with, but it's a slow-twist barrel for patched round ball. I don't think I would own a 45 caliber barrel for saboted bullets simply because there's not much variety and you have to be so careful to maintain the projectile caliber according to my state laws.
You can also add some weight to the rifle to absorb some of the recoil as well. Unless you do nothing but spot and stalk, there's nothing wrong with a little heavier rifle to reduce recoil and add stability. My S/S Encore with Laminate stock and scope probably weighs in the 8 pound range, and I don't have any problems carrying it around during hunting season. Composite stocks will lighten up the weight, so keep that in mind when selecting your rifle.
Most modern black powder rifles produced today are more than capable of killing a deer ethically out to 150+ yards if the shooter does his/her part. Most of my hunting is done within 50 yards and I haven't had any problems harvesting deer with older sidelocks and patched round ball. The key, as with just about anything, is once you get your set-up, get familiar and comfortable with it and use it repeatedly and you will know your limitations.