Building a Multi Hull
If you are really thinking of a circumnavigation then the 34 foot Benford ''Badger'' would be a far more ideal way to go than the Wharrams. If I were thinking of a circumnavigation I would build the Badger with the cutter rig. The Badgers are really well thought out design that have a proven track record and yet are extremely easy to build. I do suggest that you buy a copy of Anne Hill''s book, ''Voyaging on a Small Income'' although I am not a fan of the Chinese style Junk rig that they chose. Here is a link to the Benford sailing dory site http://www.benford.us/index.html?dories/index.html
A 30 foot Wharram would be very close to the bottom of my list for a circumnavigation.
If you don''t mind me asking, how experienced are you both as sailors and where will you be building this boat? All too often people focus on what appear to be simple to build solutions to building a boat without realizing that the hull is but one small component of what it takes to build a boat. While the Badgers are a really neat design, it may actually make sense to spend just a little more time and build a slightly more sophisticated boat that will have proportionately more resale value.