Tom tom will kill your batter because:
You will have the screen on the whole time you run it
It uses a good amount of cpu
Usually the brightness has to be all the way up in a car to see the screen well
Just get a car charger
As for a gps
Just offer the dude like 10 less than its posted for and he will take it...
Here is the 240 everyone is telling you to get (i use it also)
http://cgi.ebay.com/Holux-SIRF-3-GPS...QQcmdZViewItem
and here is a bigger but cheaper one (it does more channels, if that matters to you)
http://cgi.ebay.com/Holux-M-1000-32-...QQcmdZViewItem