My vote would be: volts, oil press, and oil temp. Also check out the Audi
4000 instrument mount suggested by the list that would mount below the radio.
You could get lucky at the wrecking yard and get the whole works off the Audi
for cheap (get the senders too).

Ken Hunter
82 Westy 1.9D
84 Jetta 1.5D
89 Mitsubishi van