I had the very same questions myself about 6 months ago and luckily, ran across some experienced guys who were able to guide me on this. I'll share what I did and what has worked for me.
Before I start with what worked for me, please know that one does NOT need the latest greatest, high speed PDA to run Exbal. That being said, I would personally avoid the Palm products because they are a dying technology. One thing going for Palm stuff is that you can power them with AA batteries. The military recently bought thousands of used Palm devices and refurbished them for field use because they use AA batteries. Horus Vision aggressively markets their ATRAG software which only successfully works on Palm but even Mr. Sammut (the President of Horus Vision) admits that Palm is a dying technology and that they are trying to get ATRAG to work on Windows-based PDA's. (That's another long story...) With Windows-based PDA's, you get a color screen and a technology that will work well into the future. If somebody else comes up with new and improved ballistics software, it will be Windows-based PDA stuff and not Palm. The only downside to non-Palm is that you will have to recharge the battery. However, this is not really a problem since you will probably use it for hunting or shooting instead of everyday computing. Once you get everything saved and set in your PDA, you can fully charge the batteries and turn the unit off until you get to your FFP. (Final Firing Point) (They actually never truly turn off so I make sure to charge mine up every few days.) If you do go with a Windows-based PDA, you will have plenty of battery power for hunting. Still, this is a personal choice for the end user. Now onto what I did and works for me personally...
First: After looking at several other guys' PDA setups, I located a used HP iPAQ PDA on eBay and won the auction. I read all the online reviews I could about the various iPAQ's and their capabilities before bidding. The one I got is a lower end version that does not offer all the perks/abilities of the $400 jobs. Still, it's color, it will run Exbal, and it still has all of the other cool stuff that PDA's offer such as calendars, scheduling, Word, synchronizing with my Outlook, etc. It cost me $100 to get the PDA, some software, and cables for power and synchronizing w/ my desktop. Not a bad deal.
Second: I installed Exbal on my new iPAQ and learned how to use it. Then I realized that my PDA was vulnerable to dust, gravel, being dropped, and water. How was I supposed to take a PDA into the mountains? Sure it works great on a clean range but what about hard field use? Luckily, another friend of mine turned me onto Otterbox products.
THIRD: I bought an Otterbox protective case for the PDA. It is literally an armored, rubberized case that you can put your PDA into. Here is a photo of my cheap PDA in the Otterbox case:
Again, instead of buying brand new from the factory, I went to eBay and found a new one for approximately $80. With an Otterbox, you don't have to worry about dropping your PDA. It has a hard, protective cover over the front of the PDA that you can raise and still use your stylus on the face of the PDA.
There is a clear plastic "membrane" over the front of the PDA which keeps it waterproof. So in effect, there are two layers of protection over the front of your PDA: One hard, and one soft.
The Otterbox provides shock/drop protection for your PDA as well as waterproofing it. Now, I can literally punt my PDA in the middle of a downpour and still use it. There are several different models to choose from. You can see their products for PDA's here: Otterbox PDA Cases
FOURTH: I realized there was still a possiblity that I might run the batteries down and lose all of my information on the PDA. This is unlikely as a fully charged PDA will run for many hours before dying. However, it's still a risk. As a fix for this, I bought a memory card online for about $20. Now, all of my rifles/loads are saved on the memory card which stays inside the PDA at all times. In the event that I do lose everything due to battery failure, all I have to do is recharge the PDA at camp with either a generator or the pickup's power and I can immediately get all of my ballistic information back for the hunt in the morning or that evening.
There are military and police snipers who have much more sophisticated setups than mine complete with direct inputs for wind meters, weather, GPS, etc. However, the "rig" described above will work great on a dusty range or for mountain hunting in the rain or snow. Hope this helps you with your decision. Good luck.