Bruce "seaworthy" and "blue water" are terms that are debated often here. But what you're describing is really coastal cruising as opposed to setting off across an ocean. You'll find most production boats are capable of coastal cruising.
So it becomes a discussion of what boat/brand/layout suits you best. There are as many opinions on what makes a "best boat" as there are sailors.
Neither seaworthiness or live-aboard comfort are tied directly to boat length. Some 30 footers are roomier inside than some 36 footers. Some 27 footers can cross the Atlantic, some 35 footers I wouldn't take beyond the sight of land.
The only way to really get a feel for the different types of boats is to get some time on different boats.
Being a confident sailor in six years is a very doable goal. Some ways to get there include taking sailing classes (I'd guess there are multiple schools on the Great Lakes), finding some local racers who need crew and finding a local yacht club that has club boats shared among members (usually small day sailers).
You might also consider buying something like a Catalina 22 that you can lake sail to gain experience. You can find these boats under $5,000 and sometimes for much less in decent shape. You can sail the heck out of it for a couple of years, learn a lot, have a lot of fun and sell it for what you paid when/if you're ready to move up.
Hope this helps,