I'm assuming when I use google maps etc it burns through the credit, guess I'll find out..
Certain things to think about (using Samsung Galaxy Ace, but that has the same Android system as you).
Turn OFF the "location service" (or whatever it's called on your phone) as that will eat your credit up by constantly telling Google where you are. I also turned off the "data service" over the 2g/3g network so things don't access using GPRS/EDGE/etc all the time, eating credit.
Use your wi-fi connection wherever possible for any type of internet access, but remember to turn that off when not in use. Think battery power
Why use google maps when you have built in GPS, which can be enhanced further by use of assorted apps?
Make sure you buy a decent sized (8Gb will be enough, for starters) miniSD card for the phone as once you discover the Android Market you may go crazy....
As far as battery life goes, I can only speak for myself. I have the wifi on most of the day, use the phone as a music player, obviously make calls/sms throughout the day, email/faceboök/assorted sites checked constantly throughout the day, you get the idea. My battery happily lasts 3 days between charges which I don't think is too bad when you consider we are looking at something that has more computing power than my first 2 (maybe 3) windows PC's.