Some considerable time ago, someone posted regarding ammo choices:
"Stick with the Big Five: Federal, Remington, Winchester, CCI, Peters." -- not necessarily in that order. Once upon a time I owned a Jennings semi-auto in 22 cal. 6+1 capacity I think. I never felt that it wouldn't at least seriously slow down an assault.
If you can place a round in the brainpain any .22 will work.
If 22 LR was what I had available, I'd get into the mind-set of emptying the gun, pretty much point-blank, center of mass. Would be nice to have a mag option, rather than having to reload a revolver.
Head shot is good IN THEORY, but in fact the first thing that goes in a defensive situation is small muscle control. You want to go for CENTER OF MASS, and be close enough that you'll hit it. (Under 10 feet). The SECOND part of your 22 cal. strategy is to develop and refine a speedy retreat.
ANY firearm serves as a potent deterrent.
No one facing down the muzzle of a 22 LR handgun is going to taunt and cajole. The ol' 22 cal. is a favorite choice for hit-men in the mob -- because it's LETHAL.
At the risk of seeming to side w/ DKS Joe Biden -- your apparent budget constraints and current availability would suggest that a used 12 gauge would be an excellent choice.
Chuck Karwan in "Combat Handgunnery" -- in the FIRST chapter -- advises: "Never take a handgun to a gun fight." You want a rifle or shotgun. I had a State Trooper, when I asked him how he liked his 1911 Colt Gold Cup compared to the standard issue Glock 22, also noted: "The side arm serves only to cover my butt until I can get to the long guns in the squad car."
Used shotgun should come easy --