I don't think there is any way to protect oneself from a huge bill if the only person who knows about the problem is keeping quiet (assuming the maintenance engineer who told him about it is also keeping quiet).
I am sure this has been done to death already in the many "group" threads but it would seem to me essential to meet all the present owners, and look at the maintenance records, and also meet the maintenance firm and ask them to confirm (in writing if possible) that they are not aware of any problems other than those listed.
The problem, as I have found out in another context, is that maintenance records are usually spread far and wide. It is common to get work done (especially expensive avionics work) by a firm far away from base and perhaps never use that firm again. The records of the work done should of course be kept but if they aren't then nobody will know about it, as there is no systematic way to locate the stuff. The only way to find out is by stripping the plane down and comparing the number on any part that carries a serial number with the s/n given on the original build documents (if you can find those) and even that isn't foolproof because e.g. a dodgy ADF (£3000 a pop) could have been repaired any number of times.
With a private owner (not ltd co.) it's just as easy to pay cash for work done.
With many avionics firms preferring to generate typed-up A4 logbook certificates (over writing entries into the airframe logbook), and with these notes being easy to "lose"....
If a plane is dodgy, the selling member keeps mum, and the remaining members keep mum (which they usually will; it's not in their interest to have a member who wants out but can't), there may be no way to avoid ketting shafted.
The one thing which should be apparent is a plane that's got a long good or a long bad history. But a huge one-off just around the corner cannot be guarded against.
All this stuff is very negative of course, and I suppose it means that the people in the group matter more than anything else, because you just have to trust them.