I had a friend with an 88 Euro M6. Bought it from a doctor that took meticulous care of it. It looked brand new with 80K miles on the clock. I'm not kidding, it was showroom perfect. Every time he takes it in for something it's $1000 minimum. Granted it's an M, however if his car is "perfect" and it still costs that much to maintain, that 633 ("with little rust") is going to be a money pit.
I'd pass unless you need a winter project car and have the means for such a project.
Well, every 6er is going to be a money pit, no matter what - they are expensive cars to maintain, period - parts can be pricey, and if you can't do the work yourself, you'll get charged out the ass to have someone repair it for you. However, $1500 is a good starting price point if the car is, in fact, as described.