more about the o2 unlimited plan
here
I believe the FUP is around 200mb a month. What you also need to make sure is that you don't have your APN settings to mobile.o2.co.uk as you will be charged for that data. The slower wap.o2.co.uk is included in the plan.
I'm using a freebie satnav on mine (with a puck) and haven't been billed any extra. Also, I don't think that you need to specifically set the browser to GPRS/EDGE. On mine, it flicks between the two depending on the strength of signal I guess. Hasn't affected the satnav anyway
And interesting, looks like they've dropped the 8120 for personal users and are now offering the 8110 which has GPS built in.