Unlocked phone on t-mobile flex pay plan

guys i am new to unlocked cell world. I just discovered what it meant yesterday. anyways, here is what i want to do. I want to get a tmoble flex pay plan with no contract. to do this i need to buy the phone out right. i have been looking on ebay for cheap phones and they many. my question is this, can i buy one and have it turned on with the flex pay plan? Is all i have to do is buy an unlocked phone and sim card then have turn it on? Lastly, Tmobile told me that if i got a side kick i would have to buy a data plan. can you buy an unlocked sidekick(or any smart phone) and use it just as a phone and text with not data plan? and how would you do it. thanks for the info in advance guys.

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Yea..but it defeats the purpose..sidekicks have plenty of features