I live up here in North Wales and I would be interested in a share in a Rib. Exact specification is something I haven't thought much about, but about mid range, I suppose would be good.
It is a bit of a mine-field, but as long as you are all adult (and that can be the tricky bit) then it can go quite smoothly.
this link
boatshare agreement
has most angles covered. The other thing is the individuals experience. If you insist that all parties must at least have completed Powerboat level 2 then it can deter the odd fender-bender from joining.
Let me know if you would like to take it further. My boat is for sale at the moment and after it's gone, then I would be very interested.
Rossi