I’ve been lucky enough to be able to fly a Cessna F172H for several years now. The engine has always performed as expected and reliably delivered a good ride.
I can only agree with everything Overandout says about the 172H but that given current issues around alcohol in mogas recommend you to stick to using 100LL. For permit aircraft UL91 has been agreed for use in the 0-300 by the LAA, however it’s not clear what the situation on this is for C of A machines such as the 172
http://www.lightaircraftassociation....aded%20AAN.pdf
If the engine in the aircraft you have in mind is low hours but high calendar time (10 years) I recommend the pre-purchase inspection of the aircraft includes a check that the mags and any other time lifed items have been serviced or replaced, that the carb float service bulletin has been implemented and if the engine is “on condition” past 10 years that AD 94-05-05 has been carried out.
There is Cessna 172 group on yahoo who might also have some views that can help you
cessna172 : Cessna 172 - A place to talk about Cessna 172's
The owner of an O-300 engined 1959 Cessna 172 has a lot of information about owning and operating the aircraft at
Cessna 172 G-BSEP - David Williams flying homepage
My favourite feature about the O-300 installation on the F172H is the really big hatch in comparison to later models for getting at the oil filler and dip stick. It makes the oil check during the walk round very easy and the occasional topping up of the oil can be done directly from a litre bottle without the need for a funnel. You also get a really good view of the general condition of the engine crankcase and equipment attached to it.