Blackberries will always use data, particularly if you haven't wiped it and cleaned out all the service books from the previous owner. It will be trying to connect to the RIM Operations Centre to connect to the old BES and BIS accounts.
If you are very careful about what you load on the device, and how you manage your connections you can probably do what you want, lots of people seem to be able to. You have to be aware though that the Blackberry is not designed to support this kind of activity. Read the
FAQs, starting with this one:
What do I need a Data Plan for? - BlackBerryFAQ