You'd be very lucky to find a used TM that has no problems, and if you have had no experiance maintaining them before, you soon will have!
Ask at the dealers as they are the ones getting the trade ins as people upgrade, and usually will give a shorth warrenty on a 2nd hand machine, and don't be scared to start small, you don't need a triple wand carpet shagger to start with. Because it's quite a jump up from a electric machine to a TM remember the most power you can get out of a 13amp socket is 3120watts I think my 13hp Tm is about the smallest machine and that develops 8200 watts at 80% of revs. then you can go up to 50+ hp.
And most importantly you need to look after them, it's not a simple electirc moter and a pump like a portable. There are valves, belt, couplings and all sorts you need to get to know and look after, unless you live near a supplier then it easy, get them to do it.
Hope that helps or at least give you some food for though.