For practical reasons, transportation and storage, a folding bike might be the best choice. Downtube ($300), xooter swift ($700), Dahon Mu Xl ($900). Swift will customize.
The most ridable of modern bike are the dutch bikes. http://www.dutchbikes.us/
If you are willing to experiment try Shaft driven bicycle http://www.dynamicbicycles.com/
A good bicycle shop can convert almost any bike to an internally geared bike. See Horizontal Dropout http://www.sheldonbrown.com/gloss_dr-z.html
comfortable seat - if you are picky you will have to buy the seat seperately.
components to look - If you can get a chain cover like on the dutch bikes that would be real nice.
size considerations - endless discussions of wheel size in the folding bike area. I would suggest you find the best bike shop in the area and buy from them, sizing is not that easy imo. You might also try a few used bikes to see what works for you.
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.