I just scanned what I believe is the blog you're referring to and the boat ended up going on it's own bottom in October '08.
Hi, I spent a bit more time reading and came across where it says a delivery crew were hired for the journey. Back in May however the blog said this: http://www.nordhavn68.com/aspx/controls/mail/viewItem.aspx/parm1/14163
Granted they were headed for some great cruising grounds, but I can't imagine not going along for that delivery. I know they were dodging hurricanes, but I was surprised to read that they had concerns about cruising in 6' seas.
I have done the entire great loop in a 75' Hatteras MY, which can definately handle 6' seas no questions asked. However, if and when I do an ocean crossing, I wouldn't leave in 6' seas. I have more of crew safety and comfort concerns, your body can only take so much and everyone ends up being exhaust. There are also more issues as to something breaking (not with the hull, but other stuff). Also, the most major thing, 6 foot seas can build to something much larger pretty darn quickly. When you're making an open ocean crossing of any distance. I once was in the gulf in 3' seas most of the day and it kicked up to 8-10' in 30 minutes as this squall came through.......scary stuff.
Most of you refer to new but I think a good used 86' Burger is the best. A little short on range but it will handle every thing else. I ran mine with two crew and also with just one over the past 7 years. It went to the med on its own bottom. It was great in Alaska. And it should be in budget. I might be a little one sided as it is listed in YF boats for sale.
There are many vessels that meet your criteria on 1-5. Your question is "...what kind of boats..." and the simple answer is a trawler. Many posters recommended specific production trawlers. Nordhavn and Northern Marine were mentioned; Krogens have also made transatlantic crossings. There are several Dutch builders whose trawlers are not only capable but have the "wow" factor. Not sure where you are specifically going on your longest leg that requires 4000nm range, but there are numerous trawlers that do have that the fuel capacity and the consumption rate that enable it. In addition to the production trawlers I've mentioned, look at custom yachts. By custom, I mean mariners who had your same criteria and had yachts built to suit their needs - layouts and aesthetics vary. If you want to be able to single hand a trawler, I recommend staying approximately in the 48-55' range (if a single engine, then bow thruster is preferred and stern thruster is icing on the cake.) If you will have someone else aboard, then the size can extend to perhaps 72'; over that, I recommend crew as a necessity if not so much for handling, but for making life easier in the housekeeping department. Buying a smaller boat to see if you like cruising is not comparing apples with apples. I recommend chartering a similar trawler to your ideal. In general I do not recommend novice boaters building new, and you shouldn't need to. You can find your right boat that meet your criteria! Judy Waldman