I've never closely followed the various discussions here in the past regarding living vans, but am now beginning to wish I had as I am contemplating whether I can adapt a van trailer for such a purpose. Although I don't recall the discussions here ever produced a definitive answer?
The other part of the original question I suppose is 'is there a maximum train weight (vehicle plus trailer) for the K9?'
Most period trailers seen behind preserved MVs tend to be as used in service - e.g. Ben Hur behind a GMC, Arrows behind a Bedford MK, Rogers behind a DT980. Whether they are braked or unbraked seems to depend on what their original spec. was.
But in this case (and to help answer the original question) what criteria do you use to decide the maximum size of trailer - be it braked or unbaked - that can be pulled by a particular vehicle?