A BB is not like an ordinary cell or smartphone. You will need a BB data plan for Internet, email, Maps, Facebook, BBM and other RIM related services. A generic plan can work for Internet, but it is not a great way to do it and you will need a workaround to get it to work.
If the device is not o2 branded, then it would need to be unlocked to work on that network. BBF has an unlock service to do that. I think the cost is less than $10 US. Don't know what the cost of the BB plan would be on o2. We have some members here so once they see this, they should be able to tell you.
Start with this from our FAQ. It applies to buying ANY used BB, not just from eBay: Buying a BlackBerry off eBay - BlackBerryFAQ What do I need a Data Plan for? - BlackBerryFAQ