Hi Mike,
I have a 91 500sl also an import, it came with a digital speedo in KPH only.
I managed to find an analogue one and tried it - it worked fine, no problems.
The local Mercedes dealer said it would not work and also said they would have to put it on the computer at 300 gbp otherwise the car would not run !! Bull**t no problems, just another Merc dealer ripoff.
N t-j