If you don't have a data plan you'll only be able to stay in T-Zones and the 3 tabs (Downloads, Web, and Account)
Don't worry about overages because in the 3 years that I have had T-Mobile, not once have I had an overage (with or without a data plan) with internet features.
I think they don't even let you get overages with the internet. If you don't have BIS it wont say EDGE in upper case it'll say it in lower case (edge) so you'll know you won't be able to use the Blackberry browser
Now...
That doesn't mean you cant use the internet through Wi-Fi.
Open your browser then go to Options > General Settings > and change your default browser from Internet Browser to Hotspot Browser and surf away