Thank you for the replies, sorry for my tardiness in answering.
The step up in size is not a necessity, although I think if the opportunity arose for the right price I would do so, the extra length does have it's advantages as has been pointed out. Genoas suggestion of the 3 - 4 year old boat is probably the way I would lean and that would potentially be regardless of size, though I wouldn't go smaller.
But the brand new engine with full warranty does appeal too, my concern with this was the possible effect it would have on resale value if it was stuck on an older hull, whether the condition is good or not.
I think maybe my best option is to wait and see what if anything is available at the time, as I said this is some months off now at least and if a real bargain appears out of season, (and considering the summer we've had it wouldn't surprise me if it did) I'd be rather foolish to pass up the opportunity, of course I may win the lottery in that time too!!
Whatever I decide, rest assured, we can all look forward to another which engine/boat is the best debate.