Irrespective of how the car looks, in particular when buying privately, with the possible exception of a fully documented restoration project, there is absolutely no substitute for a full pre-purchase inspection conducted by a Porsche specialist! It doesn't take many seemingly "minor defects" to add up to very big bucks to put it right which could leave you with serious negative equity in the car.
However, bear in mind that nearly all modern cars are going to take a depreciation hit anyway, so some negative equity in a classic is in a way akin to taking an upfront depreciation hit.
However, bear in mind that nearly all modern cars are going to take a depreciation hit anyway, so some negative equity in a classic is in a way akin to taking an upfront depreciation hit.