First, I do not recommend slotted, drilled, or drilled & slotted rotors. There's lots of discussion on this topic on the internet and on this very informative website.
Regardless what kind of rotor you're looking for, check out the sponsors on this site- they offer a good selection of quality OEM replacements.
I'm not sure of the e31 brakes on your e34... someone will chime in on that.
A popular "upgrade" is low-dust brake pads, which reduce the brake dust significantly. I used to wash my wheels every week, but I've gone a month before the same amount of dust builds up with my new pads/rotors. Coated rotors are always nice as well- they don't rust quickly, which is ideal since your rotors are mostly visible with 18" wheels.
EDIT: I have Balo rotors and PBR pads on the front. I switched to PBR Metalmaster pads and Zimmerman rotors an all four corners on my e46 and they work great! Highly recommended. One last bit of advice: you don't need to drop hundreds of dollars on state-of-the-art rotors and pads. I am almost certain you don't race/autocross your e34, and therefore will not have a need for performance braking components. IMHO. :thumbup: