I have the Garmin GPS III, older model, not the plus, and it works real well with velcro on the dash. I don't think that you need an external antenna unless you plan on mounting the unit somehwere other than the dash. Here's another link for alot of GPS units and supplies
http://gpsnow.com/gmgen.htm
If your going to use the GPS just for street navigation, the Garmin Street Pilot is great, my step father has one and uses it all the time for business travel. If you want a unit that you can also take with you for a hike in the woods, the GPS III is much smaller and more suitable for that use. The only tradeoff is the small display that can be difficult to read while driving. Hope this helps.
Scott W.
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