Welcome aboard Freddy...

1st question is are you looking only for a tow vehicle, or a 'car' that can be used daily...?
We have a (now 10+ year old) Holden Suburban which we use for towing a 3500kg boat/trailer. We also have a LR Discovery and I have towed with the RR as well. The Suburban (6.5L diesel) is the hands down winner as a tow vehicle, but by far the lesser 'car'.
One of the big Ford F-series would be a good bet. I don't think they make them anymore though if you are looking for a new vehicle.
I would expect the RR to be a better tow vehicle than the Cayene - but that's just a gut feeling... the Cayene being more of a sports 'car'.
I would also opt for diesel over petrol too - generally more torque etc. The diesel Suburban for instance is a vastly better tow vehicle than the 5.7L petrol variant. We have one of those too....
IMHO when you are towing a big boat, you want as big and heavy a tow vehicle as you can get.
In terms of biggest towable boat... I could've provided you with a bigger one that's still under the 3500kg limit... though you would need to display an oversize sign whilst towing it....take a look at "Graphite" on my website
http://www.imaginocean.net/