How do you come to that conclusion (design's being old and "squared", what that means)? You lack the necessary skills to make that type of appraisal, don't you think?
You have limited expertise and experience, in all the areas you'd like to jump into. In a nutshell, you learn to walk after getting a handle on crawling, eventually learning to run. The same is true with yacht design and boat building. Stock plan pricing will range on size, complexity, the specific designer, etc., but a good set of plans will be a $1,000 or more. Custom plans will be considerably more and based on the same reasons.
Your first yacht design would be after considerable study and closely following a known design, 15' or less. This is the usual route, which breeds successful outcomes and offers a baseline for future design improvements, that can be quantified with the "baseline". The same would be true of your first build, something small and simple, so can can learn and finish, before you get too bored. As your design skills go, so do the ambitions of the next designs, eventually you'll have the engineering confidence, to attempt a 40' cat. Ditto, building a boat of this scale. You just don't know how much effort (and money) goes into a boat of this size.
Lastly, you need to get out on the water. Beg, borrow and steal rides on as many different boats as you can. You have no idea what you really like or need, so lots of different experiences are best. Go down to the local sailing club and volunteer as crew. You'll gain valuable skills quickly. I'm on the water 3 or 4 times a week, unless up to my eyeballs in epoxy. Most of us that have made the life you dream (liveaboard and/or cruising) have done so after countless thousands of miles under our keels. Personally, I was born on a boat and raised on an island, so I knew what my destiny was going to be, long before I could walk. Living aboard sounds a lot more romantic than you might find it actually is. I've done so several times in my life and enjoy it, but it also has limitations and is a lot more costly and restrictive than living ashore. It's a choice and you don't need help yet, just the willingness to get out on the water and learn a little. My advice, buy a $500 dinghy and splash it in any local puddle ($500 dinghies don't go in oceans). By the end of the summer, your ideas about sailing, design requirements and what you currently think will be dramatically different, than it is now.
