It all sounds promising. I have towed a big caravan which wasn't great in cross winds and a 3 ton trailer which was fine but taxed the clutch a bit with it so far. But never towed a rib of any substance. They usually say weight nearer the tow vehicle is good so engines on the transom doesn't sound a great idea....However the huge length of the A frame on a boat trailer probably gives the tow car more control over the boat.
Taking into account my experience and everybodys comments I think it should be ok.
Make sure you have plenty of weight on the nose, nothing worse than the boat trying to lift the backend of the vehicle off the road. Even more critical if it's a 4 wheel trailer, which I assume it will be.
Rule#2: Never argue with an idiot. He'll drag you down to his level & then beat you with experience.
Rule#3: Tha' can't educate pork.
Rule#4: See rule#5
I owned a 90 for a few months, but soon changed it as the 90 never felt safe towing close to 3500kg. The 110/Disco/Isuzu D Max that I have now all feel much more stable especially when braking going downhill.
To be fair I've heard people say they are pretty poor and some say they are fine. Mine wasn't amazing with a 2 ton caravan, but seemed okay with a 3 ton trailer. I guess there's only one way ti find out !