Nick
50 kg that's crap! Are you sure, most cars can take more than that, only diddy wee Corsa's etc can only take 50 kg nose weights?
What kinda Beamer is it?
My E39 can take 90kg I think I have just bought a E39 98 S 523i Auto and I am working my way thru reading the manual too. Fords and Vauxhalls can take 75 kg.
I would have thought that it won't be your rear suspension set up that limits your nose weight it will be the car a a whole that limits the nose weight as it will affect the balance of the car as a whole. But I stand to be corrected. I know of a lot of caravaners fit beefed up suspension to their tow car to cope with the extra demands on the cars suspension when towing. Might need to go down that road I'm afraid.
Might be worth writing in to the Q&A pages of Practical Caravan or another caravaning magazine for advice.
Andrew