Can't help with the boat under £8k, but my Seasafe (essentially a Humber O-pro's grandad) with a Yam55 weighed in at around 440KG fully tanked up. Granted the engine was a 2 cyl, and the O-pro is a lot beefed up to take a 4- stroke 90 now, so could probably ad a couple of hundered KG for that.
Strictly speaking I don't need brakes on mine at the moment, but I decided to go for a braked unit in case I bought a lighter car in the future. (Although the materials that make cars lighter seem to be being offset by all the gadgets that we all now seem to need, so I may find I've wasted my time!
Having said that I've once had to do an E-stop with it on the hitch (numptie did the usual "it's got a trailer so will be doing 20mph, so it must be safe to pull out"). I was bl**dy glad I had the braked trailer.....