All you really need on a BlackBerry is the unlimited internet access. This will give you just about everything you need. If you want to send text messages instead of E-mail then you may want to get a text messaging plan as well, but that is completely up to you.
There are also several other options, such as if you would like to use the BlackBerry as a modem for your laptop then you can get a tethering package that will allow you to do this. And also there is a higher internet package if you decide to get on BES (BlackBerry Enterprise Server) for your E-mail. Most times BIS is enough if you are just using POP3 email but if you want your E-mail, Contacts, Calendar, Tasks and Memos all to sync without using the wire then you may want to get on a hosted BES solution and get the BES data package from your provider.