Alasdair, welcome .
the big scay numbers being quoted can be improved on by going for a secondhand boat. The downside to that is unless you know outboards and are happy to take the "was it abused?" risk, it may not be the right option. Having said that, I;m sure you could get a traded in boat from a dealer with warranty.
As for upgrading the O/B, that's not too difficult, as there is a standard spacing of mounting holes for a huge variation of horsepowers - I helped someone literally unbolt a 50 and put a 90 in it's place. Both engines were Tohatsus so we didn't even need to change the fittings on the end of the throttle & gear cables, and the electrics plugged straight back in!
I think the 6.5m has come from "to seat 6". I might offer a disagreement - the boat I helped up the HP on is a 5.3m and has seating for 5. What you need for bums on seats is beam. You could feasibly fit 6 seats in a 5m, but not a "2m beam" boat. Humber for example do three beam versions of their O-Pro (first one that came to mind, others are available!
One option might be to keep an eye on the "cruises" section, and see if anyone needs a crew or scrounge a lift. Then you can travel in variations on a theme "in action" & get a better feel for what will suit you best.