I wouldn't recommend anything that didn't have at least
- Bluetooth
- Windows Mobile 5
- QVGA (320x240) touch screen
- Lots of memory options (SDIO, CF)
- Field-swappable battery
for both backward and forward compatibility. The rest (phone, GPS, WiFi) can be added
Ad libitum. Many brand-new but superannuated iPAQs are being offered on eBay from dealers who get pallet-loads of last year's Cingular (or whose-ever) offering for pennies, then resell them - usually in the original packaging. Search for
ipaq brand new, then sort them according to descending price.