The disc can be bought from the Berlin local authorities for €6 including postage and it arrives within the week (other places charge more.)
Per caravans you are limited to 80kph on autobahns unless your van has been tested by Tüv and given the Tempo100 badge. Problem is we don't register our vans separately from the tug, but from what I have heard the testing stations seem to be happy to use the CRiS code.
Anyway it isn't a problem. You will be limited to 80, lorries are limited to 90 so if you do 80 you will get blasted by horns all day long. What most people seem to do is to get into a string of lorries all doing 90 and so will you. Legally you cannot overtake them, but if you go slower they get nasty, so the Police will leave you alone.
You will also need a disc for France - in fact moreso for France as many more cities and towns now require them, You go to this web site
certificat-air.gouv.fr
fill in the form and add your credit card (CB) number and attach a copy of your V5c and it arrives in a week. It costs €4.51 including postage and like Germany it is permanent for the car with the registration shown on the disc.
Finally, don't even bother getting the sticker if your (diesel) vehicle is not Euro6.
Oh, and another final thought. The instructions that come with the French disc are confusing about where you place the sticker on the windscreen. In practice you just need to attach it so that a officer of the law can see it when standing in front of the vehicle. To be compliant with UK law it must NOT be within the swept area of the wipers, so top nearside is the best bet.