The things that usually wear on cars are servicable items. Ball joints, calipers, water pumps, drive belts etc.
If the car has been looked after (and service intervals adhered to) there is no reason why the serivable items will be in any less good condition than on your 172 cup.
An 77k williams that is serviced correctly for example will have a cambelt, tensioner and idle pulley in better condition than a 20k 172 cup.
The problem is that people skimp on servicing, and that other people buy the under serviced cars, find problems and sell on again with more propblems.
A fully serviced williams will be as reliable as a fully serviced 172 cup, all things being equal.
The trick is therfore to only buy a car that has been serviced at a main dealers, and has been serviced with oil ever 3k and all other items at the correct intervals.