I use a laptop hooked up via serial cable to a garmin III from 1997. It works like a champ. I use the Capn with free raster charts from NOAA. The Capn has a 30 day trial so you can test it. I realized I could just set the Windows clock back and use the program forever. I felt guilty and bought it a while back.
I have every NOAA chart for the east coast loaded. If I zoom in, it automatically selects a higher resolution chart. If a rough inlet with shoal and 6 foot waves, Amy monitored the laptop giving me headings while I white-knuckled the tiller. We got through it thanks for the Capn and no one losing their heads. A usb GPS would be a good choice, nice it uses the laptop battery (less cabling). Anything that is NMEA compatible should work with most programs. Is NMEA the correct acronymn???
Anyway, good luck.
Here is our trip using the laptop > www sundancecsc com/mac25mods htm
Replace the spaces with dots.