There are so many good old boats that have or can cross oceans, some needing extensive mods, some not that many, that it really pays to do your homework. My old "sailing instructor", the late Jean LaCombe, had crossed the Atlantic no less than 5 times singlehanded in boats from 18' ( Hippocamp. a wooden double ender he designed and built) to the Lapworth 23 I was on. He also sailed a 21' Golif in an early OSTAR (singlehanded transatlantic race). He later designed and built a 25'er (Yang) that he cruised to Europe and back. Most people today think a larger boat is the way to go, but so do the costs. For a couple, boats from around 24' - 25' and up seem to have done the job. There are so many old fiberglass boats around I think a little work would make a lot of them suitable for long distance cruising. For those who get seasick, try sailing on a trimaran or catamaran, although they'll be perhaps more expensive. The old British Heavenly Twins 26 / 27 has circumnavigated for example, and has twin aft double cabins. Searunner trimarans, or Tri-Stars (31' and up) make excellent long distance cruisers too, even though most are made of wood. They have excellent safety records too. Know that the movement on a multihull can be somewhat different from a monohull. they tend to move more at the ends, but you'll never have a long roll as with some monohulls, or much of a hobby horse motion. You'll also have access to some very shallow areas that monohulls may not be able to get into.