I could see a .38 Special being a good choice, but I've seen some used Ruger P-95's for under $300 lately. I'd take one of them over any "questionable" .38 revolver bought online. I've also seen Security Sixes, Speed Sixes, and an SP-101 for less than your budget. Others report used Smith and Wesson's for around that price - I've never seen one but I believe these guys when they say they're available in some areas. Point is though, there are better choices IMO.
Also, if you're talking pure HD, you might consider a pump action 20 gauge shotgun. Long arms are generally easier to shoot more accurately than a handgun and the 20 gauge will hit hard at indoor distances.
Those who say start with a .22 have a point. If possible, it's a great plan. If the situation is someone needing a real defensive weapon RIGHT NOW though, I would quickly move to something bigger if at all possible.
Still, worst case scenario, I'd still prefer a friend be armed with a .22 handgun or a reliable 10/22 with some high caps than be totally unarmed.