Or get a Baur!
In the more humid countries rust is an issue with the inner parts of the sunroof. And you can have mechanical trouble but that usually is the result of someone fiddling with it by someone who doesn't know what he's doing, there are some real experts out there and the sunroof system is not unique for BMW's but used on pretty much all contemporary German cars. I'm no expert either, but at the local dealership there's a specialist on the topic. If it works fine, make sure the mechanism remains clean and lubricated and it'll last.
To me a sunroof is a must on a closed E21, especially over here where E21's with A/C are rare as hen's teeth. One thing though, you loose some inside headroom on the car, believe at least 4 centimeters, so if you're a taller person the sunroof might be in your way. That's why E21's equipped with a sunroof came with slightly lower seats.
