Back in the 90's, when I was young and stupid I bought a 28ft sportscruiser and put it on triple axel trailer. At the time the Land rover Discovery V8 handbook said it could tow 5T (I didnt bother reading any of the important stuff about how it could tow that weight - air brakes etc ), so I loaded up the rig and set off with my new toys. The whole rig weighed between 4.5 and 5T depending on kit.
Getting it going wasnt a problem, the chipped V8 had plenty of grunt but anything over 40mph was brown trouser time and going down hill was heart attack time. Stopping it (even with upgraded brakes) was a lottery, on several occasions the whole car would simply slide along like a curling stone and that was in the dry. Slipways were the scariest, the moment the weight of the boat went over the lip, the whole rig started sliding.
I learnt my lessons during that 1st season and bought a smaller boat during the winter. I love Land Rovers and still tow 3.5T with a Range Rover but IMHO regardless of whether its legal I wouldnt want to tow anything over3.5 - 4T with a Land Rover again. Have you looked at Mini Artics ?
If you are still looking for a compressor check out some of the 4x4 off road suppliers, they use on board compressors and tanks for locking diffs.