I doubt whether you'd be able to find/buy a C172 with the Thielert conversion in any case. I think Cessna was very, very relieved that they did not yet deliver any to customers when Thielert went bankrupt. Diamond ended up with a whole lot of airframes, mostly DA-42s, ready for delivery except for the engine but they're now delivering these with their own Austro engine, and Robin (who delivered the DR-400 Ecoflyer) is in receivership right now. In all cases, getting spare parts for anything Thielert powered was impossible for a while, and is very expensive but at least possible right now.
I tend to agree with Bert. Buy a relatively cheap two-seater now, preferably one who is JAA certified (for instance with the Rotax 912S) to run on mogas. There's actually a few aircraft like that which would almost fit your budget. Just for reference, I have Flyer from March 2009 here, which has the roundup of all light aircraft for sale today. Included are, for instance:
AT-3 R100 Club, euro 67.000. (
| www.at-3.com)
Aerostyle Breezer-C, euro 37.800 (
Startseite)
CSA SportCruiser, UKP 50.820 and up (
Czech Sport Aircraft)
Dova DV-1 Skylark. Price unknown. (
www.dovaaircraft.cz)
Evektor Sportstar RW, USD 105.000 (
Welcome to Evektor - Aircraft Producer and Design & Engineering Company)
Funk FK-9 MK IV, euro 51.685 or other Funk airplanes (
FK Lightplanes)
Issoire Aviation APM-20 Lionceau (
www.issoireaviation.com)
Meteor Sky Arrow, USD 103.300 (
SKY ARROW web site)
Rans S-7 Courier, USD 85.000 (
default)
Tecnam P2000 Sierra (various models;
Tecnam)
Of course, you would want to be very careful with renting these planes out for flight training - they are probably not as robust and long-lasting as a 172 or PA-28. But you could consider renting them out to post-PPL pilots who are hour building towards their CPL. With an appropriate checkout of course.
In any case, buy an aircraft that fills *your* needs. Not the need of the flight school who might want to rent the aircraft from you. If the flight school would be able to give you sufficient business for a price that would allow you to run your aircraft relatively cheaply, they would buy the aircraft themselves.
Oh, by the way, USD 2.60 for a liter of 100LL is by no means exceptional. I think the current price at my home base (Rotterdam) hovers around the 2.30-2.40 euro per liter right now.