If you're looking for a new device, I think you can get Essentials on the Z-22 from Palm ($99). It has 20MB of free storage available - 32MB total but 12MB is reserved for system operation. No support for expansion cards. Essentials needs a minimum of 8MB, but you need over 10MB for more than a limited experience. Forget about putting much else on the device, and probably no big contact lists, appointment lists, etc. It isn't a device that will grow with you in a medical application.
Personally I'd lean toward an E2 ($199) which supports SD expansion cards. For the extra Franklin you'll have some growth - currently up to 2GB of SD space if you need/want it. For another $100 you'll get WiFi with the T/X ($299)- my suggestion for a device with longer-lived functionality. And finally yet another $100 gets you into a LifeDrive ($399). Epocrates has a mid-$200 deal on a Treo 650 package (including Essentials) if you sign up for a 6mo. cellular contract.
Around $300 is the entry point for Pocket PC (my preference). These are all list prices.
