I started using an older Palm that I bought second hand a few months ago. I was using the Palm Tungsten E, and I really enjoyed it.
My experience with PDA phones is not exactly pleasant. I would suggest splitting it up and having two pieces of technology. Granted, it makes for carrying things akward, but it just seems to work effectively for me, your situation may be different.
If you aren't very savvy with technology I would recommend a Palm based OS. They are great all-round, but they appear to be a little easier to use.
I'm about to advance to an HP iPaq, as we're rotating the PDA's in the family. I'm passing my Palm to my mom, who isn't the most technologically advanced individual. I'm picking up an older iPaq (since budget is limited), so I'm hoping I'll enjoy it.
Honestly though, if you're just using it for notes, and not for applications like music or video, then be wary of salespeople selling you a powerful handheld computer. For simple notes and dates, you should never need more than 64 mb of RAM. On the other hand, if you want internet access through 802.11b (wireless) then you'll probably have to go for a more advanced model.
Finally, if you can get a business model, then consider going that route (provided it's power matches your needs) as they tend to have a longer battery life than the standard models.
Just some suggestions, and good luck with the purchase!