Factors to consider, IN ORDER, when buying a used bike:
1. Is it the right size? That's a yes/no question. Would you buy shoes that were a size too big or a size too small?
2. Is it rideable now? No = basket case. Price should be REAL low because owner obviously doesn't care about it. It'll always cost more to make rideable than you're thinking.
3. Everything else. Not even worth talking about until 1 & 2 are right.