Before you buy a used BB from any source, you need to read this: Buying a BlackBerry off eBay - BlackBerryFAQ
And once you find one you like, check the specs and see if it has GPS. Keep in mind that you will need a BB dataplan to get maps and there are no free map apps that give auto reroute and turn by turn commands.
You will probably be happier with a portable GPS device rather than a cell phone with GPS.
And, the 8707 does not have GPS.