Can I buy a used blackberry, then simply put my SIM card (AT&T) into the Blackberry to be able to use it? I want to be able to web browse, and my account has unlimited media, but I don't have the Blackberry package. Can I get by without the extra $$$ for a Blackberry package?

No you need the blackberry package.

Welcome to the BBForums.

Read this about buying a used BB... Buying a BlackBerry off eBay - BlackBerryFAQ

But you will need the BB Data Plan, yes.

And read this about not having a BB dataplan: What do I need a Data Plan for? - BlackBerryFAQ

A regular MediaNet plan is usable, however it is a very poor substitute for a BB data plan. You will need to D/L and install Opera Mini - or at least try to make it work. Some have, others have not. You will not be able to receive email on the device without a BB data plan. IMHO, if you are going to get a BB, then spend the few extra dollars a month and get the BB data plan - $29.99 on ATT for the personal plan. You will like it much better.