If i could only have one gun & it had to be under $300, i would go with my mossberg 500 shotgun with the 28" hunting barrel & the 18.5" home defense barrel. I bought mine very used some years ago, but i'd bet you could find one in your price range. Further, i would think shotgun ammo would be least likely to be restricted by any future government regulations as it has such a history with hunting (what did Cheney shoot his hunting buddy with, yep
). Additionally, many areas require lots of paperwork or permits for handguns while they cut shotguns a bit of a break (so i'm told). A pump shotgun is also very simple, almost instinctive to operate. You can get a lot better shotty for $300 than you can get in a handgun.
If i had $300 for a handgun, i'd look for a good used revolver. I have seen some nice-looking, used ruger .357's for not too much more online. edit* J&G Sales has what looks like some police trade-in revolvers in your price range http://www.jgsales.com/index.php/smith-wesson/revolver/cPath/16_211_431
, but i've not bought from them myself. I have heard some good comments from other buyers.