BMW R1200RT/R1250RT Forum

I bought a low mileage 11plate RT from an indi recently. I had similar issues, in that I couldn't get the preload to change, either on the display nor any movement detected from the shock. Turns out shock was fine, it just needed recalibrating which a BMW dealer did. Cause was bike had been sat with...

Nice bike.
Keep an eye on used forums such as eBay, you may find the box that you're looking for at more reasonable money.
I've bought several that way for various BMW bikes and never been disappointed