Dave_G

I think what Simon is trying to say is that if the self levelling suspension was working correctly, then when you hooked up your trailer the rear wouldn't sink like it is now but would automatically raise itself back to the correct height and therefore the car would be much more stable and safer. I don't know how to check what type you have but I think they were all called Nivomat. Perhaps ringing Don Kalmer or similar might help there.
The oil cooler will probably depend on the kind of towing you will be doing. Dragging a heavy load up long steep gradients is going to be hard on the transmission but I'd think normal motorway work should be ok. I know the T5 has an engine oil cooler as standard.

Hope this helps.

Dave