Marlowe, You really will need Farm Trac+ if you're going to use Farm Trac Mate. Farm Trac Mate is designed for the Pocket PC and is primarily meant to be a field data collection device. Basically you enter your records into Farm Trac Mate and then you synchronize the data with Farm Trac+ and use Trac+ to print all of your reports.
With regards to GPS, the HP iPAQ is probably the most used Pocket PC when it comes to GPS mapping. The main reason is that it is easy to get the cables you need to hook up an external GPS. If you are going to use a Compact Flash GPS, then make sure that the Pocket PC that you are buying has a Compact Flash and a Secure Digital Card slot. You will use the Compact Flash Card Slot for the GPS and the Secure Digital Card slot for a card to log data to. Of the currently available HP iPAQs, we see that the 2110/2115 seem to be the best on the market now.
One last thing, if you plan to make maps with the GPS for use in Farm Works, you will need Farm Site Mate on the Pocket PC. If you plan to make maps other than coverage maps (planting, spraying, tillage, etc...) such as scouting, boundary, tile and other maps, then you will also need Farm Site which runs on the desktop computer with Farm Trac+.
You may also want to take a look at our FAQ webpage. In the "Site Mate and Guidance Corner" there are a few topics on different Pocket PCs and other devices.