Don't know what the tankage is on a C42, but a watermaker might make up any shortage. Since you want extra cabins and only plan to cruise for a few years and not push the boat too much, what about a used charter boat of appropriate size? Would be a lower cost, but would need more upgrading, but probably cheaper overall.
In South Africa doing lots of boat stuff. Departure north from Cape Town around December 15th. Arrival in the Caribbean around the beginning of February, after 5300 miles or so.