Hey Simon...good to see you back here!
I agree that if you need to ferry more than a couple of people, plus gear in a busy, choppy harbour you'll need more than your minimal 7' inflatable and a 3 hp kicker.
If you plan to take it with you on deck then find the biggest one that you can stow on your foredeck and still sail the boat. This will likely put you up into the 9-10 foot range, and maybe up to about 6-8 hp. You'll also be looking for an outboard davit at that size...
However, without a trailer this may now become problematic to transport if you plan to take it home with you between trips....
As ever, it's gonna be a compromise.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"