Blackberry 8700g question..?

Hi, I am interested in the T-Mobile Blackberry 8700g. I was just wondering if I needed a Blackberry plan in order to have this phone, or if a regular voice and unlimited texting plan would be sufficient? I don't need the blackberry plan, but this phone is affordable for one with a QWERTY keypad. Also, I was considering buying a used one on ebay. Does T-Mobile penalize you for not purchasing their phones from them directly?