A good point, and one I forgot to ask the dealer (had thought of it, honest!). The dealer emailed:
"This motor was sold to a sailing club and had very little use, it is a "2005" but experience a powerhead fault, it was returned to the manufacture and had a complete new powerhead (engine) fitted and came complete with ! years manufacturers warranty, it represents half the cost of a new one, and it is like new."
I agree needs clarifying whether the warranty is a new 1 year one (which I presume would be better) or merely the final year of original. Will also ask what caused the powerhead failure. My main concern as a beginner is buying a duff engine second hand, but not keen on £5k for a new one. I thought this may be a good compromise.
Thanks for above replies, v useful info.
Apologies, but you don't seem to have picked up on the grommet problem mentioned above. Do check it's been sorted and sorted properly on this engine otherwise that powerhead will have to be removed again in very short order - it's about the age it starts to show and it's a time problem rather than a number of hours used problem - affects all Merc/Mariner 4 strokes 40-115hp of that vintage.