I think this is going to be a tough one. The E34 was not designed with any of the Nav features that the nicer E39s were, so you are almost certainly going to have to do something aftermarket. I believe Bavarian Autosport has an in-dash Nav system that will fit in an E34 but it is only text directions and I dont think it is GPS, it just feeds you line by line instructions. I am pretty sure it is made by Clarion. You can head over to bavauto.com and look there. I have been using the Garmin GPS 295 with lots of sucess. Its primarily for pilots, but I use it in the car as well as the plane. Garmin does make ones like the Street Pilot that are specifically designed for use in the car, so that would probably be a better option. They mount right on the dash and their display's are excellent. You can spend anywhere from $300 to $1800 on these units, all depending on what your needs are. Good luck and let us know what you end up with!
Brian