Have you considered the cervelo RS. Its not marketed as WSD, but fits me better than any WSD since it has a 73 degree STA. Smallest is a 48 cm (w 650c wheels) but with a sloping top tube it may work for you. You mentioned hip/back issues, and it has a nice tall head tube so you don't have to have your bars too low.

When you ask what to look for on a test ride, when I test rode this bike I cried cuz I never felt so comfortable AND powerful on a bike in my life. It felt like I imagine a good custom should, handled well, flew up hills.