Rev Counter

Is it possible to fit a rev counter to a series 3? I have no idea how hard the engine/gearbox is working and would like to know.

Yes.

Plenty of search threads on it


Goes onto the alternator W terminal

You wire it to the ignition, it divides the ignition pulses by the number of cylinders so get a 4cyl one.
The w terminal alternator output will be a multiple of the rpm but probably not an integer multiple so will need calibrating. These tachos are available but are more expensive and are really intended for older, mechanical diesel engines.
Depends if your series is petrol or diesel.