Simply buy the phone (whether new or used), and enable WiFi. You'll have full function of the iPod section and all data services (email, IM, app store, browser, etc) providing you're within range of a WiFi network.

You do not need a sim card to use these features (with my old Rogers 3G, no sim is required if you're on WiFi).

I can't remember correctly, but if there is NO sim card, I think there's a window that may or may not stay open saying "no sim installed," correct me if I'm wrong.