Speedometer and Idle questions

I've been looking at an 85' Nissan 300ZX non-turbo. So far I've only encountered two problems with it, and the one may not be a problem. When the car was idling, the Tach showed a smooth idle at about the correct RPM range for the car, but the car seemed to vibrate more than normal or at least more than the other 300zx's I've seen. The guy says it has something to do with the lower idle. The car has an exhaust which is alot bigger than the stock exhaust and he's done a port and polish. He says it has lost power in the lower RPM's, but has gained power in the higher RPM ranges. Could this be the cause of the vibration, is the vibration something to worry about?

The other problem is the Speedometer. It sits at a constant 0 mph and the odometer has gone with it. I don't know if the cable is bad, or the speedometer is actually broken. He said it's the cable. Is a speedometer something hard to fix? I don't have alot of automotive experience, so I was curious if this would be something I could do on my own, or would I be better off getting a shop to fix it?

I have a 91 Nissan 240SX and I can relate to the speedo dieing.  As for mine, the motor fried and I just went to a junkyard and swapped speedos out and everything works fine.  Check the cable first, the sensor on the tranny second, then the speedo motor last.  As for the idling problems the mods might have contrubited to the low idle and vibrations, but I have a vibration when my idle goes below 500 rpm.  My problem is the 02 sensor and some clogged parts.  Check all that and clean/replace as needed.  You shouldn't need to worry about the vibrations unless its very rough or continues at higher rpms.