well its up to the buyer to ask what on the car is factory or isn't. The thing about these cars is that most become enthusiasts once they buy the car. A guy who never thought about an aftermarket wheel or ever bought an expensive wax changes a great deal of his behavior when he pulls the p-car trigger. I can assure you that most will not be too happy to know that everything on the car didn't come from factory, until of course he reads how much even the replica wheels are going for these days. I don't think you can buy replicas for what I paid for my factory Carreras. Although I got a screaming good deal. Better to give full disclosure--when asked--before the sale.
p.s.
I want a C4 as well, they make great grocery getters. I want red with tan leather. I just dont have the room or drive enough to justify a second car. But if the prices keep falling like this it will be too hard to pass up. i test drove one last summer 07 and teh price had fallen probably 15K in that time. I'll just store the Boxster off-site during winter.