If you look at the PDAs being used they are mostly quite old, as the screens are easier to see in daylight. Suggest you look at what people are using where you fly, as having a local 'expert' can be very useful. See if they will let you 'test drive' in a car.
Note that if your PC doesn't have a serial port you may need to provide one for older PDAs. USB/serial converters seem to have a patchy track record, internal cards (for desktop PCs) and PCI cards (for laptops) seem to generally work properly. And if the PDA will write a declaration suitable for the EW to an SD card that's good as well - you use the PDA to write the declaration, put the card in the EW and turn it on. The other approach is to simply put the correct pilot & glider details in the EW and do paper declarations. Remember, the latest one is the valid one.
Note also you want a PDA that takes 5v in (I have a very old one that takes 3.3v!), as that's what the Microrecorder supplies out for a PDA, so long as it's connected to the glider's power supply. For some bizarre reason the Flarm I have supplies 3.3v out!