Kars used to take ages (over 10-15 mins) before it heated up, although I was running mine where the oil pressure switch goes in the B14. I always took that as a sign not to thrash the engine till 15-20mins into the journey...
I know when we looked at these last time that VDO make several sensors with the same thread and internal resistances, but with differing length of sensing bits, I'm guessing a shorter one would heat up quicker (smaller, therefore less energy needed to heat it up) As long as it fits in the sump, I wouldn't worry, they all run with a 180ohm resistance.
As for temperatures, Kar would run between 70 and 90 once it was warmed up, depending on the type of driving I was doing, but I don't think ever went above that. In the winter it took ages to get off the pin, and also when the engine was stuffed it didn't rise much from what I can remeber either.
I miss my gauges

, could always tell what the engine was up to.
EDIT:: 400's with the info centre display the word "COLD" until the temp is above 50 degs, dead useful, and the reason I'm ditching my temp gauge when I get Kar running again...