500 hours is probably 25% of an engine, so you'll need an engine fund. It's also 10 50 hour checks, and your assuming nothing untoward is found.
Be aware though, unless it's a busy school, you will struggle to achieve 500 hours, what with WX cancelltions, no-shows, etc. For example, our 172 was first registered in 1973, and has TTAF of just over 4,000 hours - tha't's an average of 121 a year! I know that this is probably not representative, but if you're going to be spending £15k - £20k you will be getting a v. high houred machine, that will need some TLC
If it were that simple, I'm sure most people would be doing it, and why would the school not just buy the plane themselves at those rates, and have an assett?
I think you've already recognise that it may be just a little too good to be true!
Good luck though if you do go for it, and enjoy the flying!
DW.