The things that I would consider are.
What the car would be used for.
Cost of maintenece when the car goes out of warrenty.
Cost of insurance.
Aftermarket parts avaliability.
Cost of the aftermarket parts vs cost of factory parts.
I'd go on a GTR and GT3 forum and look around there as well.
Again. I'd go for the new GTR vs a used Porshe.
1999 328i Sport Package, Premium Package, 5 speed, Alpine white.