Hi welcome, as with any purchase, check for legitimacy of what your being offered.
Buy on condition rather than price.
Check the known faults of CRM's, all of which are extensively listed in the search area.
Remember, the value of your purchase may go down as well as up

it's an odd market out there at the moment.
As a rule of thumb, MK1's are obviously the oldest, and can reflect this in their condition, but a good one is still viable.
MK2/ 3, had many changes that gave greater performance as standard, and were less prone to owners neglect, but only marginally.
AR,s have greater potential to be money pits, [ do a search], though again a good one should give excellent service, with all the usual caveats.
Others may not agree wholeheartedly with my summing up
Mike