Hi there,
You should be able to use either a device that runs either Windows CE or Palm OS, as either will allow the sharing of text, word and excel files (Palm OS uses software called 'documents to go'), and I believe that there are useful family history programs for either.
What I would make sure is that the PDA you get is reliable. I have used 3 different ones so far - an HP iPaq (Windows CE), a Palm Tungsten E and a Palm Tunsgten T2 (both the latter using Palm OS). The T2 I have found to be the most reliable of the three. Both of the others have had a bad habit of crashing on me.
Secondly, I would ensure that it is easy for you to use. The Palm Tungsten suits me, but I would try to 'play' with a few first to see what suits you best. As part of this, I would look at the the battery life and whether it takes ordinary batteries or has a rechargeable, lithium battery built in. How you enter data, etc.
Thirdly, I would try to make sure that you get one with a decent amount of RAM (no less than 32MB). Also, that it has the ability to have its storage capacity extended by either a Compact Flash card or SD card.
Finally, I would think about what else you might use it for, and factor that in as well.
I hope some of this helps - if you want to ask me any questions on it, just send me a personal message on it, and I will be happy to offer any advice that I can give.
Cheers,
Clare