That's not so unusual though, is it? People (for as long as I can remember) talk about buying a second hand car as "buying someone else's problems". If you don't want to risk that, get a brand new car! I've (when shopping with my own money) always bought "sheds". Never spent more than £1500 on a car in my life. Last one cost me £600 I do always assume that what I'm buying is bad and will need a lot of work (and I'm rarely disappointed)!
People can just get fed up and want a change. The car that (for them) is too much trouble to put a new DMF into, will be new and exciting for its new owner, and nothing will be too much trouble for a while.
I also tend to go for obscure, complicated cars (Big old Alfas and Citroens) that nobody wants. I'd pay a lot for the equivalent Ford or BMW, but with those, everyone and his dog who reckons they "know a bit about motahs" will have had a go - sometimes rather ham-fistedly. With an old Citroen or Alfa, they are generally neglected but not bodged - at least, not to the same extent.
Anyway, as far as the OP is concerned, just don't expect miracles. The AA or RAC will find existing faults (some of them at least), but nobody is going to be able to tell you whether your (say) alternator will pack up next week ,or your "wet belt", the week after, or your starter, the week after that. All they can really do, is report on the bits they can see "today". Old cars have an almost infinite way of screwing you over though...