1. Any one of those could be the truth. What does your contract say?
2. Without a BlackBerry data plan you won't get any BlackBerry data services. Email, built-in web browser, BBM, Facebook, etc. If they don't offer BlackBerry data plans and use a proxy of some kind instead, you may never get these services.
3. Google Maps is BlackBerry data. The stand alone GMail app is not, nor is Opera Mini. Dunno about IM+.
4. Only you will know how much data you use on a regular basis. Streaming radio uses a ton of data. I use between 90MB and 120MB per month myself. Your usage patterns could be different.