Unlocked iPhone 4 on T-mobile plan?

I'm planning on buying a unlocked iPhone 4 from ebay. However, I want to know if the unlocked iPhone 4 would;
- Call and text for free?
- Use internet (wi-fi only)?
- Download apps from the app store?
- It is a smartphone, will T-mobile charge me extra or charge a smartphone value plan on me if I use it for their network?

I'm currently with T-mobile and have the family plan in effect. I would get an android, but I like the iPhone much better and I don't want to pay extra $$$ for every month for the smartphone plan since I barely use 4G internet outside or inside. I already asked many people around me and tried searching it online, but they all have different answers.

Yes but you won't be able to send or receive pics better off getting whatever the phone service its under prepaid SIM. Say if it's an att iPhone just buy the SIM prepaid.

Check out http://www.iphonefort-mobile.com/ for additional information on using a iPhone on T-mobiles network.