Just wondering, as far as timeframe goes, lets say your route is 20,000 miles and you average 100 miles in 24 hours (4 knots and hour). It's rough but that would imply 200 full days on passage. 3:1 might work, or 800 days. Is that your estimate? Speaking of time frame, you mentioned the boat has crossed the Atlantic a few times prior to you. Do you know how that went? How long have you owned it? Do you know your boat intimately as in have you done all of the expected sailing, cruising, anchoring, navigating, inspecting, servicing, troubleshooting, repairing, and retesting on it?
Do you have a whisker pole and a mainsail jibe preventer?
How about mast climbing gear? We have a Switec - a one person, self ascending operation in a pinch.
And spares, spares, and repairs! Your boat is almost as old as ours and I know that most of our original systems and equipment have expired, failed and been replaced in one way or another. As Yngmar stated the toilet will fail so do you have a spare pump and rebuild kit and a few joker valves? Speaking of stuff that expires, the diaphragm on most diaphram pumps and bladders in your accumulator are only good for a few years. Those are a few examples of stuff that will probably fail. Have SPARES and the skills to do the repairs. If you have the gear, might I suggest Nigel Calders best seller, Boat Owners Mechanical and Electrical #3 or 4 and the shop manual for your engine with exploded diagrams.
After seven years of ownership we are still heavy coastal cruisers but just about every system on our boat has a live redundancy or a spare ready to go. Same goes for ground tackle. I've hauled the ground tackle up manually twice after a fried 20 year old windlass and a failed clutch followed by freefall (all 260 feet!). Does your windlass allow manual recovery? Have you tried it, and if not, have you done it from your primaries?
And what about your dinghy and trusty outboard? If inflatable, is it hypalon? Is it a RIB? I.M.E. PVC is junk. Tropical sun will destroy any dinghy quickly; do you have chaps? Do you have a means and the abilty to lash it to the deck? We have had good reason to carry spare spark plus and spare carburetors and the right tools (or the more complex rebuild kits). A patch kit, spare prop and extra air pump are not bad either, especially if you have the correct cotter pin.
Speaking of propulsion, what shape is the auxillary in? What do you know about major service prior to your ownership? Do you know how to perform all service interval tasks? Do you have a dual Racor or just the stock single non-visual fuel filter? This will matter when the crud in the 23 year old tank gets mixed into the fuel. Do you have a couple impellers ( and a puller sure is handy) and a complete spare raw water pump and a thermostat and related gaskets? How about all varieties of sealant/adhesivesneeded, and speaking of handy how about including butyl tape and a SplashZone kit?
I could go on and on about tools and spares parts, service parts, bulbs and fittings, but I'm certain you have this covered. A few that one never seems to have enough are anodes (rudder, saildrive/propshaft/prop, water heater/calorifier, outboard, possibly refrigerator, others and PFD repair kits. Speaking of anodes, I make marine water heaters for a living and you are risking premature failure if you have a watermaker as the source and don't know how to properly maintain it. I just saw one that failed in under 3 years of seasonal use.
And to leave you with one more random thought, if any of the potential worst case scenarios happens, are you and your partner prepared? Have you done all the dry runs? I'm certain the scary stuff is covered but what about a plumbing issue like losing power and not being able to get fresh water out of the tank?
A circumnavigation is serious business. You just can't be too prepared but the key is to make sure you do it and to have fun and make lifelong memories for everyone. Be safe and ENJOY!!