I would highly recommend against a mechanical oil gauge, definately go electrical.
Everything else is a personal choice.
But for practicality, the only gauges worth doing are;
volt/amp
oil temp
oil pressure
exhaust temp
air/fuel (if you use a wideband o2 sensor)
boost (is you have FI)
transmission temp (more for ATX)
fuel pressure (if you have modified the fuel system or added nitrous)
nitrous pressure (if you have nitrous)
