In the meantime, I have a (new) friend with an E89 35si who is also a bus driver. For his job he has loads of hotels to recommend that are also affordable. I also got my best friend warmed up to help me organise this trip. He is an ex-police officer, now working with Interpol. He has many international contacts and he is also member of the motorbike club "Blue Knights", an international club for (ex)-police officers. He has loads of experience with road trips. He also already escorted car clubs with his bike, which he might do for us as well. Last one was a road trip with oldtimers, a line of cars for 5 kilometers bumper to bumper...
Distances per day will depend on average speed, so at higher speeds we may cover 500 kilometers, at lower speeds maybe half. We will also foresee at least 1 day to rest. The whole trip, including rest days, will not be longer than 1 week, to keep it feasible and affordable for many.