A 9mm, .40 SW and .45 ACP have about the same muzzle energy. If the bullet stops in the target in a vital area, any one will do the job. The trick is hitting a vital area.

To maintain proficiency, you have to shoot 200 rounds or more a month - 100 to get back to baseline, and another 100 to work on improvements. That's $30 for 9mm, $60 for .40SW and $90 for .45 ACP, half as much from Wally World, much less if you reload.