So I found a cheap 1970's Apollo Mk II frame set on craigslist for $50. It appears to be in fairly good condition (I emailed the seller to ask for specifics).

I've also heard some people say that doing a conversion is a waste of time, as they don't usually turn out too well with noobs like myself. I've got a good idea of what I'd want to do, but am not sure if I have the wrenching skill (or tools). Plus, I'm not really keen on spending more than $300 at the moment... I really don't know if I can keep the cost low and still get a decent bike.