I'd skip the monocog and get a Karate Monkey. I know it would cost more but the monocog just seems so limiting.
Really though, if you don't need a 'good' mountain bike, just build up a beater SS. Do it on the cheap, drop $200 or so max and call 'er good. Seems to me that you should be able to snag some used/free parts at the bike shop and make it work.
Basically I'm saying that if you want a beater, don't drop $500 on it.