Re: gps and power ?
If you want to use the GPS for short period of time, the batteries will be sufficient enough, but if you want to run it for long time, then better option is to use external power.
As far as the "RS-232" interface is concerned, be aware that most of GPS devices (including Garmin) operate only 0-5V what in fact has nothing to do with "true" RS-232 levels. But because for domestic applictions this level 0-5V, that is, was easier to generate, most of contemporary PC, laptops, etc. will read these devices..
And for that reason (no proper RS-232 driver) you don't need external power..
Regards,
IanP