As I read your post it really sounds like you are looking for two boats; one to learn to sail on and one to live on and long term cruise. Boats that are big enough to live on, especially if described as 'the biggest boat that I can single-hand', are usually too big to learn on if you intend to learn to sail well.
Ideally, if you really intend to learn to sail well, (and not everybody cares whether they actually learn to be good sailors but that's another topic) then I suggest that you would be well served buying a small (23 to 30 foot max with 26-28 feet being more ideal), tiller steered, used but in good shape, responsive, fin keel-spade rudder, largish production run, ideally fractionally rigged, sloop.
You can own a boat like that for a couple years, sail the living daylights out of her and sell her for pretty much what you have in her. You will be years and many dollars ahead of the game in terms of learning boat handling skills and what it takes to own and maintain a boat. The deductible for the repair costs for single accident with a boat big enough to live on could well exceed the entire cost of owning and learning on a smaller boat.
When it comes to the biggest single-handers that you can can handle, the traditional rule of thumbs were based on displacement and not length. Before the advent of modern deck hardware, and lower drag hull forms and rigs (easier to handle) the rule of thumb used to be a range 2 1/2 to 5-6 tons (long tons) per person. That would suggest that anything over about 11,000 lbs would start to press the convenient limit (roughly a 38 footer max). With modern gear and efficient rigs that number can be extended so that it is possible to handle a much bigger boat, but as boats get bigger they become dependant on higher levels of skill, lots of luck, and much better equipment than a new sailor is likely to have.
Lastly, the Hardin's were a miserable boat to sail. To me they are a characture rather than good sailing boat. So while they may shiver your timbers, I suggest that spend as much time as you can, sailing as many boats as you can, of as many types as you can, and I suspect when you are done doing that you won't have to ask us what kind of boat you should buy and will know why the Hardin is probably not a great choice for whatever you want to do with a boat.
I do not mean this as a put down in any way. We all had to start somewhere. I think that I completely understand where you are coming from. When I bought my first 'live aboard' in 1973 it was a totally inappropriate choice that simply captured my imagination.
Last edited by Jeff_H; 01-14-2009 at 11:24 AM.