'Lo Gone - I'm not an expert, but someone discarded an elderly PalmPilot onto me last Xmas - at first I thought that it was an amusing toy, but now I don't know what I did without it.
I really don't believe that you need all the latest bells and whistles, like barcodes or Net access or 16MB RAM or colour screens - the core functions are the ones that you need rather than games and add-ons and all the PDAs have those. I think almost all can interface to your PC for backup and other stuff and you really need that.
I'd say the main choice is whether you want an actual keyboard (bigger) or whether you can live with screen tapping for data entry.
Why not get a Palm M100 for a start? It's the cheapest of the excellent Palm's, has all the core functions and if you want a proper keyboard or decide that you want to upgrade in 6/12 you'll have no trouble selling it.
Good luck!