When you say cheap and plentiful ammo that leads me to believe you're going to do a heavy amount of shooting with it. If this is true you'd be a fool not to get a service gun (IE: Glock/M&P/XD/CZ/SIG). If this is not true then a smaller/cheaper gun would be appropriate. I'd say you need a Glock 19. If your budget is $400, scrounge/save up an extra $50 and find a used one.