Thanks for coming back to me with thoughts and my apologies for then disappearing and not acknowledging the fact that I was indeed getting feed back and suggestions!
I think it proves point that I might be looking at a big ask, , large outboards usually stay with the boats they were paired with unless exchanged early and at nearly new price or swapped out much later when the engine is getting pretty well passed its sell by date.
I will sit on the fence for a little longer, I have put a number in the spreadsheet next to the 'engine' box and made an offer on a RIB on that basis. It looks like the project may not get off the ground so I am going to stop worrying about it.
If it does, I promise will come back looking for clues with a better brief.
Thanks for the help.