Personally I'd buy one, use it for your own use and lease it back to the school where they are always double booked etc. I wouldn't buy new and if you buy for the right price you will get your money back. The yellow and black one suggested by 206 jock isn't such a bad idea and remember because its not hydraulic does not mean that it flys like a hydraulic machine with the hydraulics switched off. I have flown both hydraulic and non hydraulic machines and sure the hydraulic is much better but you are working to a budget here. One other consideration is that the flight school may be reluctant to lease back a non hydraulic machine, might be worth checking first. The machine advertised is in good nick, I bought a Raven 2 from the same guy who gets them from a pipeline inspection company in Germany, so they're not worked hard and no I have no interest financial or otherwise but can tell you he has had the machine at least since November last year when I bought mine.
A raven 1 or Astro will burn around 17 gallons an hour and insurance will vary on hull value and whether or not you lease it back for trainingbut on a spend of around £115k expect to pay around £7k witha n extra £1.5k if the lease back is required. You can also just add it for a few months if that was suitable to you and the flight school.
If I can be of further help just ask.