I think a mechanical oil pressure gauge is more worthwhile and more responsive so you should already be plumbing oil in to the back of your dash is your considering an oil temperature gauge, admittedly you'd need an extra oil line.
You could also ask why you'd want to know your oil temperature. Oils are less temperature sensitive than they used to be. It's the effect on the oil's function we're interested and you'll have installed the pressure gauge to measure that more directly. Oil temperature is related to coolant temperature. The exception being if you had a sudden increase in friction - it's doubtful the temperature of the sump would react to that quick enough. I suppose it might be a means of monitoring the transmission.
+1. For a road car I don't see need for oil cooler in the UK, unless you have fitted a sumpguard.