I ve seen the searider you are referring to on boats and outboards, 6k seems a lot of money. Did he say which charter yacht it was tender to, always worth doing some reaserch.
Re the prop I think if all the blades have been sheared off it has probably damaged the gearbox. I would get it surveyed or checked over by a marine engineer.
You could try draining the gearbox oil and see if it feels gritty like or if bits of cogs come out the drain plug its buggered! Does it select gears at the moment. Nice looking boat if I remember rightly, if it was a bargin price I'd risk it but its well over priced.
This 2003 5.4 came on today:http://www.boatsandoutboards.co.uk/view/F51829/
Probably better value.
Worth waiting for someone like Tue or Pete7 to answer this post cos they know what they are talking about!