You gat into some of the fine points
of boat design here. Many boats have crossed oceans that were designed as daysailers. I would look for a smallish cockpit, a companionway that is entirely above the combings, robust standing rigging, a deep hull and full keel with an attached rudder, redundant bilge pumps at least one of which is a large capacity manual pump, large cockpit drains, windows and hatches that won't get blown in by a breaking wave, you must be able to carry everything you need for thirty days, so a lot of storage space. You won't be anchoring each night so you need a good cooker with mechanical pot holders.Oh yes and most important, the ability to flush over board. Holding tanks are for inshore waters.I am sure that others have their criteria.