Are you just buying a twin fitted RIB for the one trip?
Seems overkill to me but then I ain't that Rich......
Been boating off Cape Wrath and along the north coast, launching is quite difficult in places, (try Talmine for the scariest slip I have ever been on) but its not the worst place to go unless the weather is bad.
Been there in a 5.3m Humber with a single 60hp and and a 2.5hp aux, diving as well!