Nevada Space Grant
This is Garmin V. We picked ours up for $450. It's a fairly high end model. This is the model we use in the chase vehicle. It comes with a serial adaptor, and a car power adaptor.
The standard cords may work for you; our setup requires a 2.5 mm phono plug for the Garmin V instead of the normal serial. We use this setup before we started to run APRS+SA. You need a phono plug end to your garmin cable if you're going to use it with a kenwood radio.
This GPS unit lasts for about 24 hours on one set of batteries.
The LCD display allows you to cross check radio, laptop, and GPS location to make sure everything is working correctly.
It allows for waypoints (this helps on hiking to downed balloons)
At $450 it's a little pricey. Any other GPS unit that supports a cable that will go to serial with an LCD works just fine.
It's not very well suited for carrying around when looking for balloons.