Spray 28, 33 as offshore boat
To begin with and in all fairness, I am not a big fan of Bruce Roberts designs in general. But of all of Mr. Roberts work, the Spray series strikes me as the least well founded design brief. If you study the original Spray, you would conclude that Josh Slocum sailed around despite Spray, not because of Spray. Josh Slocum was the consumate seaman and got away with practices that no sane person would advocate as a good idea today (such as sailing around the world doing celestial navigation with a dime store clock that was missing a minute hand- a 5 minute era is 300 mile goof at the equator)
Getting to the specifics of the design in question, (and I know as I type this that someone is getting ready go ballistic and cite the name of someone who sailed one of these turkeys around the world), while you may be able to sail a Spray 28 or 33 single-handed across an ocean, the Spray 28 or 33 would be less than ideal choices.
Here''s how I see this, to begin with these are shallow deadrise (vee bottom) hull forms. They have minimal ballast for boats that are this shallow and heavy. This means that you would expect that these boats would have a very small ultimate stability (the angle of vanishing stabilty or in other words the angle at which the boat prefers to be upside down rather than right side up) and a comparatively quick motion. The snappy motion is quite hard on a crew, and if you are single-handing, high form stability boats such as these make a bad choice. They will tend to wear you down more quickly and will tend to be less forgiving of a heavy air error.
When you talk about the size boat that can be easily single-handed, the way that you should be sizing a boat is by displacement. I say this because displacement sets the size of the sail plan that is required and displacement controls the loads on lines that must be handled.
While people can and have handled much heavier boats solo, a healthy person, without exceptional physical conditioning, can handle a boat with somewhere around 14000 to 15000 lbs of displacement (with 12000 lbs or less being more ideal for a single-hander) without relying on power winches and other assists, and without getting excessively ground down in heavy air. Above that displacement range, in order to be a reasonable single-hander compromises get made and those compromises can often affect the boat''s safety and sailing ability both in heavy going and in light.
When you think about the realities of boats like these, you have the same size spars and sailplans, the same dock fees, close to the same maintenance, same building costs (equal weight equates to equal materials), same difficulty of handling, without the the advantages (greater seaworthiness, speed, and comfort) of a larger boat (a 33 to 35 footer for the 28, and 38 to 40 footer for the 33) of the same displacement.
These are very blunt ended designs with enormous wetted surface and very inefficient foils. This results in boats that neither go to windward well and which also go downwind very poorly. While most of cruising is reaching, there are times when these poor sailing chacacteristics (especially going to weather) mean a hazzard to safety and an extreme reduction in comfort. They also require more sail area in heavy going to maintain steerage, which is not a good thing. It also means a lot more motoring and stores at times when a better design would be comfortable sailing.
Then there is the matter of building a boat yourself. I love building boats. It is a really fun way for me to spend my time but it almost never saves any money over simply buying a used boat and a home built boat, no matter how well done does not result in as good a return on your dollar as buying an older used boat of respected design and fixing it up.
Which brings us to the second part of your question. There is nothing so tragic than a person who spends years of their life building a boat with mediocre design. I have watched this up close and it tears your heart out to watch when it happens. I cannot advise you too strongly to look for a better design. If you like Bruce Roberts work, I would look at something more like the shoal draft version of the 345 for example (instead of the 28) or a 370 A or D (instead of the 33), which should be a less expensive, more seaworthy and easier to handle boat than either of the boats that you are considering.
I really like the work of Dudley Dix http://dixdesign.com/designs.htm
Mr. Dix does a wonderful job of modeling his hulls to provide a seakindly motion and a reasonable preformance. My few exchanges of email with Mr. Dix and one brief conversation suggest a gentleman who really cares about the people who build his boats and would go out of the way to be helpful throughout the process. The set of drawings that I have seen were beautifully perpared and seemed quite complete and well thought out. I would direct your attention to boats like the CW 975 [with 140 degrees of positive stability and an easy to service and construct (as well as to add a self steering servo) outboard rudder] although I would add a vertical skeg to support and protect the leading edge of the rudder in a grounding.
I like the work of Yves Tanton. He is a very creative designer.
The late Charlie Wittholz, for whom I once worked,designed a number of glass over plywood and steel designs that would be good boats for your purpose. His family still sells his designs. They are located in Silver Springs, Maryland.
No matter whose design you pick,I would suggest that you look at designs in the 32 to 35 foot range and in the 10000 to 12000 lb displacement range, with a low vertical center of gravity, reasonably low form stability and wetted surface, and an easily driven hull form.