I'd recommend you give it a good research first. That said, I would recommend a PalmOS device, as the free software available is great. Here is a sample runthrough of some of the free software on my PDA currently:
Apps:
BigClock (clock, stopwatch, calendar, etc)
CSpotRun (liteweight DOC reader)
MedCalc (formula calculator)
MedicalMnemonics (mnemonics)
Progect (project tree outliner)
Tarascon (drugs. ePocrates was a bit better technically but they were deleted off my PDA after they changed their privacy policy to start selling my individually identifiable information to drug companies and other interested parties, after originally promising they wouldn't.)
Reminder (countdown reminder)
Diddlebug (quick note draw/reminder)
Hacks:
MiddlecapsHack (graffitti over the alpha/number line to make a caps)
ContrastButtonHack (PalmV: map an app to the contrast button)
McPhling (switch between apps without going to launcher)
VirtualKB (custom mappable keyboard popup, with nice cut/copy/paste/multi undos, and allows grafitti while keyboard is displayed for pre 3.5 devices)
FontHack (replace fonts by apps or across the system. The useful one set is "Tall Fonts" and "Tall Fonts Bold" as they use the same amount of physical space as the built-in fonts, but are taller and easier to read since they use the extra space that is normally reserved over the capital letters for the international language accents)
Good luck and have fun --roo