Just my $0.02
1) I knew someone who traveled from Novia Scotia to Massachusetts in nothing but a sea kayak. So, it's not so much a question of getting exactly the "right boat", but being realistic about what life aboard will be like during an extended voyage on different types/classes of boats.
2) Avoiding problems like storm damage or sinking is not so much a question of boat type/size as it is a question about sailing/navigation skill, understanding the types of hazards you may encounter, and trip planning.
3) Experience has taught me that there is no such thing as a boat that does't need any work, especially on the used market. Budget accordingly, so that whatever you end up purchasing, you have at least a little money set aside to get her seaworthy.
Sounds like you're preparing for a great adventure! Good luck!