Excel for pocket pc lacks the Solver and Goal Seek functions, which is something that would have kept me from buying one had I known that.
I got a Thinkpad T20 for $600 used off of ebay (384mb ram, p3 700, 12GB drive, DVD and Win2k). It's not all that heavy, lasts 2 hours plus on a charge (which is about half what my maestro will on a GOOD day). I like it a lot better and use it more than my pocket pc.
Otherwise, if you don't mind the bulkyness of an axim, I don't see why not get one of those, especially at your budge. A CF slot is a big plus for me