Is it the supercharged "petrol" V6, or the TDI engine? That would make a difference, though I'm sure the supercharged engine is no slouch.
If you're looking at vintage trailers under 20' there are a number of trailers with gross weights under 4000 lb. For example, the '70s Argosy Minuet 20-foot models had a gross loaded weight rating of about 3500 lb, with claimed dry weights around 2500 lb.
On the 2007 specs (located here at Airstream's site
, may be useful for you) it shows the max gross weight of a 16' International at 3500 lb, and I know people who comfortably tow one of those with a BMW X5 3.0l.
An option for new trailers might be the Sport line (in the US at least, I'm not sure about their offerings for Europe.) Max gross for a 22' Sport is 4500 lb.