I think finding a 930 that would fit your requirements might be hard. The "good" ones are working their way back up.. Seems $30K is where the nice ones begin. Under 30K means lots of mods, or hard lives.
In general, the 930s, if left stock, were reliable. However, you just about have to just about drop the motor to do a plug change for instance. Plus, they are 20+ years old. Like any 20+ y/o car, items wear out and need refreshening. Turbos wear out. Vacumn leaks occur. CIS inhection too.. (I read issues that some CIS components for the Non-Turbos are getting harder to find.. not sure if this is true for Turbo cars though). Given great complexity to begin with, I would imagine greater complexity to maintain.
I've only driven one 930, and it was slightly modded. What a rush. Loved the boost bong
. Only a 4-speed trans up through '88, '89 had the g50 5 speed.