My own experience was that I started out in a 15' x 24" boat and within a few months, I was looking for something longer, narrower and higher performance. I'm your size (6', 175#, but should be 165) and have a relatively poor sense of balance, but quickly adapted to less stable boats. Now I consider my 22" wide Pintail to be a wide, stable, reassuring boat. The rest of my "fleet" are all between 19 3/4" and 20 1/2" wide. Of course, they vary in stability, but one of them - a Betsie Bay Aral - feels just as comfortable as the Pintail, despite being only 20.5" wide. Nick's Night Heron and Bobby Curtis' Sea Spirit are other 20"-21" boats that are quite stable, though vastly different in volume. Eric Schade's Mergansers are nice boats that are in the 22"+/- wide range.

If you build a wide boat, you'll probably find that it hampers your ability to learn to brace and roll. As a general rule, they don't handle rough water as well, either. It will also be slower than a similar, but narrower boat. Ultimately it limits your paddling development and can become a frustration. Wider boats typically have high decks and a lot of volume, which simply aren't necessary for someone your size, unless you're planning on doing multi-week camping trips.

If I were to make a recommendation, it would be to build something in the 17'-18' range with a 21"-22" beam and medium volume. That should give you all the performance you desire without hampering your development or being too difficult to adjust to. It will comfortably hold enough camping gear for a week. As you stated, there are large difference in stability and handling characteristic in the many designs that fall into that range, so once you have chosen a few appropriate candidates, you should probably solicit feedback on them if you cannot find one to try.

I guess the bottom line is that I don't know many people who bought or built narrow boats and wished they hadn't, but almost everyone I paddle with started out with a wide boat and ended up buying or building something narrower after paddling for a relatively short time.