Is it on German plates at the moment? If so, you need to make sure you get the Fahrzugbrief and Fahrzuglange documents from the vendor. These are the documents which are the equivalent of our V5C. You then need to get a UK MOT for it (might need to swap out the rear lights for ones with the fog light on the right side, but that's it - the KPH speedo is perfectly legal.
You then insure it using the VIN number. Then you need to take the German registration documents, UK MOT and VIN-based insurance to a DVLA office, where you fill in a form pay about £40 and they take all of the documents from you, for processing, which might take a couple of days. You'll then be able to collect the documents from the DVLA, or have them sent to you. At this point you'll get the age-related registration and a certificate of registration, which is a temporary document. Your V5C will come in the post in the weeks after this. If there are any concerns about the vehicle they do reserve the right to ask you to present it to them, but this has never happened to me.
I've done this a few times and it's always been easy to do and the people in the DVLA offices have always been very helpful.
