What would be a good older Blackberry phone to buy?

The reason why i am asking this is because I am looking for a older Blackberry phone that will work with T-Mobile since they are cellphone provider and plus i do not want to spend $600.00 dollars for a brand new phone and have to pay on the phone for the next 24 months to since my contract is going to be over with them in May 2014 because i am planning to leave them when the contract date is over with