If you are having medical problems you may not be able to shoot anything larger than a .22 short!
With ML rifles, There is no need to shoot "Large slugs " for the animals you say you may hunt. Depending on State Hunting regulations, you may be able to use as small a caliber as a .40, shooting a 95 grain round ball for hunting. Some states set .45 as the minimum caliber, and others have a minimum energy requirement at 100 yds. Check your regs.
You control recoil several ways using ML rifles. First, you shoot REAL BLACK POWDER, and not the stubstitutes. Second, you chose the granule size carefully. Normally, small bore guns ( under 50 caliber) are shot using FFFg powder. But you can also use FFg powder, which produces less chamber pressure, and therefore less recoil, than a comparable load of FFFg powder.
Third, you pick a gun that has a wide buttplate, and/ or a recoil pad. The width will spread and felt recoil over a wider portion of your shoulder, where a narrow buttplate will will hurt more.
Fourth, you use a gun that weighs more, and has a heavy barrel for the chosen caliber. Barrel weight helps to keep the muzzle from rising during recoil, which then puts any recoil forces directly back into your shoulder, rather than having some of the recoil lift the muzzle and slam the comb of the stock into your cheekbone.
Fifth, you can learn to use a different technique in shooting your gun, which spreads any recoil between both hands and shoulders, rather than all the recoil being taken by your dominant shoulder. See: [url]
www.chuckhawks.com/controlling_heavy_recoil.htm[/url]
Because most hunting shots are going to be within 50 yards, there is no reason to be using heavy powder charges in your gun. Accurate placement of a RB is far more important for a clean kill than more powder. The Round Ball (RB) is very inferior ballistically, and sheds velocity very quickly once it leaves the muzzle. Much is lost in the first 20 yards. What goes faster also slows faster is a basic concept of Physics that controls your choice of powder charges in these guns.
In your shoes, I would pick a .45 caliber rifle, weighing 9 lbs. or more, and shoot a 50 grain charge of FFg powder, under a .440 RB with .015 patches, and Wonder lube on my patch. Your first gun can be a percussion side lock, but as you get better at all this, flintlocks will suck you down to the dark side, and you will join the rest of us, making sparks with rocks-- because we can, and because we take an added measure of pride when we take game using a flintlock!
Enjoy ML shooting.