I've run Windows 8.1 on 2 different laptops for astro stuff and everything you mentioned has worked fine. A 64bit Windows can still run 32bit applications so that's not an issue. You do hear people complain about compatibility for some things on Windows 8.1 but I've never had anything not work. It's also by far the most efficient and stable version of Windows yet in my experience (so the opposite of Vista
). If you can't live without the old start menu just grab Start8 or one of the free ones, it's not a big deal.
Astro applications I have tried and have worked include:
PHD and PHD2, EQMOD, Stellarium, DSS, CCDSoft, PixInsight, Atik Artemis Capture, Astrotortilla, alignmaster, CCDInspector (just the demo), Focusmax, StarTools, BYE, and I'm sure some more that I've forgotten.