I was told that getting a PDA with the Palm platform is preferable to a PocketPC b/c more medical-based programs are available in Palm software compared to PocketPC. I don't know if that is true or not, but it may be something to think about or ask around.
My own suggestion would be to pick out what you like best within your price range. I am perfectly happy with my relatively cheap Sony Clie, and I haven't had any problems with it and it is fully capable to support the programs I want. My roommates have Tungsten W's and like those but they were much more pricey than my palm and we use them for the exact same things.
Consider buying a memory stick, and I would recommend buying screen protectors to preserve the screen from getting scratched and dimpled from the stylus.