How reputable a VCA member? Buying from a VCA member is a plus in my opinion. The typical VCA member browses this board and is aware of any problems and solutions to them. They typically take good care of their cars and treat them like family rather than simple pieces of machinery.
When I was shopping for my viper I wanted a completely stock vehicle. Now I sort of wish I had purchased one with an exhaust and headers. These are things I would like to do but am hesitant because I will eat 100% of the cost of these items when/if I sell the car. And like it or not, having modifications to a car brings the value down. Stereotype and quite often FACT - A modified car is an abused car. This can also be a double edged sword. Many people out there that enjoy modifying their cars, also enjoy keeping them as pristine as possible.
Quite frankly, it's nearly impossible to tell who is a good viper owner and who isn't. You can try to base things on age, wealth, etc...but it never truly works out. Take me for example. If I were selling my GTS and had an exhaust and headers on it I am sure you would be hesitant to purchase it. Why do I say this? Because I am a kid in many folks eyes (25 yrs old) and by no means independently wealthy. But in actuality I wash my car more than I drive it. Use the most expensive cleaning products and waxes I can get my hands on, and when working on my car, I take care not to even leave marks on bolts and screws I undo. I do everything by the book. I am 100% ANAL. On paper I don't look like a good viper owner, but in reality you would be hard pressed to get a car that is less than flawless from me.
Go with your gut, do some research on the individual and the car and make your decision. If you like it BUY IT! Headers and exhaust are nice things to have already on the car. In a month when you get that itch, you're gonna have a hard time dropping 1200 for an exhaust and 1500 for headers. Not to mention the couple weekends it's gonna take for you to install them.