Howdy Rick,
Thought about this quite a bit and you might want to think about a used Marstrom A-cat. It won't be as good as some double handed but from what I have been given to understand it is the best of the As for big-boys and will really fit your bill perfectly for the single handed side of your question. And for when you want to double hand it should be adequate given Karen's size.
It will also satisify the requirement of perhaps being the last boat you ever have to buy. And I can also tell you from experience that light boats are great for all of the rest of the logistics aspects.
http://www.marstrom.com/administration/Boats/acat/index.asphttp://user.tninet.se/~zdt420t/acatstuf.htmhttp://www.boatshow.com/A-Cat-Marstrom.html 
