My early thoughts on this would be that it is probably a C172P and that is in its favour, as the "P" model has a different and better engine than the "N" model, or so I am told. It is all to do with the camshaft, apparently, as that on the "P" model usually lasts for the duration of the engine, whereas I believe they can "go" before on earlier "N" models. Also, TT seems low which might indicate extended periods of inactivity which is not good for an aircraft engine, so some assurances on this aspect of the aircraft might be worth seeking. A new engine is likely to cost in the region of £10,000 and may involve your having to contribute your share if the monthly sub does not already involve a sinking fund for such expenditure.
I bought a 1981 "P" version about a year and half ago with around 6,000 hours for £31,500, but I believe good examples are becoming difficult to come by, so £39,000 may be the current going rate for an excellent low time example.
£55 per hour seems a good rate, but this is before the addition of the monthly sub. I find it costs about £60 per hour to operate, but this will depend on the number of hours flown.
I hope this might help, if only to kick off the dicussion and, of course, a search on PPRuNe may turn up more information.
Best of luck with your investigations and on going negotiations.