Thanks for all the comments, everyone.
We're keeping an open mind, but as I read more of Beth Leonard's book it's pretty clear that being able to maintain the boat over time is as important as the initial design. I've rebuilt a boat before and re-bedded all hull fittings, and replaced standing and running rigging, and it's easy to forget that all has to be done as well on a longer cruise. In relation to Beneteaus and Jenneaus, it's worth considering how easy they are to work on and maintain as well (when compared to dedicated and perhaps older cruisers).
Leonard notes that it can take two years to find a good long-range boat, so of course we should start looking.
It looks to be a rainy weekend, so I've lined up four boats to take a look at on Saturday:
1983 Rival 41 (of course)
1982 Moody Grenadier 134 (44 foot)
1988 Hylas 44
1998 Jeanneau Sun Odyssey 45.2
I'll let you guess which has the highest and lowest asking price, and we'll be comparing the boats along the lines of this thread and Leonard's recommendations. If it's not a complete downpour, I'll try to bring back images.