I too am new to the ZR-1 having just purchased mine on April 2, 2005. (Not that it was a big day or anything!!
) I was originally looking for a C4 but the more I researched, the more I wanted a ZR-1. Mine had 53k on it when I bought it and I paid less than 20k for it. Was it a steal? I think so. It has a couple of scratches on it and the interior is not perfect, but I have spoken to both of the previous owners and a lot of receipts came with my car. Has my car been pampered? Sure, but it's also been driven in the rain and was a summer daily driver for almost 8 years.
Bottom line, I think it just really depends on what you want to pay for the car. As for the differences, I don't know of any problems specific to a 1991.
I'd be happy to check one out for you in my area if you need any help.
Good Luck!
Jim