Thankyou all for your contributions. The problem is that if this is not to be a white elephant, it has to be able to persuade my wife and daughter that they wouldn't have been happier if we had done the journey in our modern Merc (highly specced E-class, with all the toys / creature comforts), or my Bentley; and at the same time make me not have too many regrets that I have been forced to leave the 550 in the garage.
Here is my current thinking after reading all your comments (and others along similar lines from contributors to the Pistonheads forum).
Mondial or GT4 are out - I reckon the back seats just aren't going to do the business for anything other than short journeys, so the family wouldn't be happy; and I would still hanker after the missing 4 cylinders (pure prejudice I know, but there it is).
My personal preference would be for the 365 - all had manual boxes and the details / fittings appeal to me more that those on the 400/412. Compared to the 456 and 612, the 365/400/412 have much more of a classic feel to them (365s even have carburettors!) as well as being significantly slimmer (important in the UK); and I think the body is drop-dead gorgeous. But I don't see the family getting as excited about it as I would, and the first time it threw a tantrum of any sort, that would be it.
A manual 456M has so much going for it, but I fear that the rear seats wouldn't keep my daughter happy long term and the lack of modern amenities would kill it for Her Indoors.
So, my current thinking is that the only Ferrari that stands a chance of keeping the family happy, is a 612, because it offers a combination of adequate accommodation, reliability and creature comforts.
I am not yet convinced, however, that it is a car I really want to own.
But thanks again for all your thoughts, which have been most helpful - this really is a very useful forum!