As the others have already stated, the bike will need to pass the TÜV test. To get through the test, the headlamp must be converted from RHD to LHD (I don't know whether the R850R is the same as the 1150GS, which could be coerted very easily by a dealer withour changing the whole headlamp assy.) and the speedo should be in km/h. The bike MUST be in original condition. If you have a non-BMW screen or an aftermarket exhaust, it will fail!!! Every accessory that is permanently fitted to the bike in Germany must have a so-called ABE (General permission for use), or alternatively, if TÜV approved, needs a separate conformity certificate.
Other than that, you will need a general Certificate of Conformity for the bike as mentioned by Bahnstormer and the original logbook.