Caliber need not come into play until after the ability to hit your target is obtained. With proper fundamentals you can shoot any caliber. Shop like you do for shoes... try 'em on first. If the gun doesn't feel "right" in your hands, you'll not shoot it enough to become proficient with it, in which case you may as well buy a ball bat, and save some money.
Use the MK III to gain proficiency in the fundamentals, stance, grip, sight alignment, and trigger squeeze.... THEN shop for a larger caliber gun.