No experience with engine replacement, but if as you say it's running fine, then why replace it? I really do believe in the saying "if it ain't broke, don't fix it" I'd be inclined to get someone other than your usual engineer to come in and give an independent assessment of the engine condition.
As for the saildrive seal, my boat is also an ex charter boat, previously used on the west coast of Scotland. Don't know how many hours the engine had run, and still don't as that part of the instruments has not worked for years, but it is now thirteen years old and is running very sweetly and the charter work stopped after eleven years.
At nine years old and just after I'd bought the boat I had the sail drive seal changed along with the sail drive bottom bearings and shaft seal. The old sail drive seal looked and feels just like brand new after it was removed, and I've read stories of people who didn't change theirs for as much as nineteen years. Volvo have to put a figure on the longevity of the seal, and will naturally choose a very conservative figure and for which I think seven years is exceeding conservative. As for the bottom shaft bearings and seal, the evidence was clear on account of having water in the oil.
On the other side of the equation, doing those changes now while you are taking an income from the boat probably means you can offset the costs against tax, so heads you win and tails the revenue loses. Good luck !