Ballistic software won't run on just any "PDA". You will need either a Pocket PC or a Palm if you want to be able to utilize the software and data in the field. Just about any of the lower priced Palms will run the software and you can get them as cheap as $20.00 on Ebay or new for $60 - $75.
Check "This Link"
for the Exbal software. They have a version that runs on your desktop PC or you can get a version for either Pocket PC or Palm. This is the same software that Nightforce sells and it's "good stuff". I wouldn't go shooting or hunting without it.
You really will need a weather meter
of some kind to do it right. You can check "This Link"
for some info on Kestrel
meters. You don't need the best and you can get meters that do the same thing the Kestrel does for less money. Google and search are your friends.