My initial thought was a S&W Combat Masterpiece, M15 / Pre-15 K-38. Really I'd take any good used K-frame I could find locally that passed Jim March's Used Revolver Checkout (see that thread stickied at the top of the revolvers sub forum if you think you might like a wheel gun). A S&W M65 like the ones Summit Gun Broker has, or a M66 which you can find locally at or slightly over $300 depending on your location. Sticking with wheel guns, The Ruger Speed/Service/Security Six series, SP-101, and the GP-100 which is also sometimes senn in that price range are also great guns.
In autos, I'd like to look at the Kel-Tec PF-9 when I get back from overseas. I already own & really like their P3AT for a deep concealment piece. If you like Da/SA the metal frame S&W autos are excellent as well. The Walther P1 is another good one, and as already mentioned even standard velocity 9mm is nothing to sneeze at. For $50 more I think a new Stoeger Cougar is worth looking into. I'd have to get a little first person eval time with both the Cougar and P1 first, but they're well liked here on THR so that bodes well for them. I've owned a S&W Sigma, and while not my first choice they're certainly reliable and have great ergonomics. Before I left the big box stores like Sportsmans Warehouse were selling them for $300 new. If I needed a $300 gun right now they're available & fit the budget.
Speaking of the Cougar, I'd like to clarify a bit. Beretta did not sell the tooling. Stoeger has been owned by Beretta holdings for several years now. They simply shipped the tooling from Italy to Stoeger in Turkey. It's the same gun, but the reduced labor costs in Turkey has breathed new life into it with it's much lower price. From reviews over in the autos forum here the Stoeger made Cougar looks to be a great gun at a great price.
In the end a good .357 Mag / .38 Special revolver or 9mm semi auto would be my choice. The ammo is widely available and inexpensive, which is important if on a tight budget. Take a look at the many good suggestions in this thread and find a gun you like that fits your hand.