Assuming you mean a Garmin GPS III Pilot be aware that there are several software versions. Garmin's
download page will show you what versions there are and what they can be upgraded to.
If you just want a cheap handheld GPS suitable for aviation use the main thing to look for is speed limiting. Some GPS's would only work at slow speeds so that they were OK for walking or sailing but not for aviation. This of course enabled the manufacturer to have a low-priced mass-market product and a high-priced aviation product.
I use a
Magellan 315 with the Datasend aviation CD and find it very simple and easy. (The 315 is the same as the 320 AFIK)
Mike