I have a question regarding T mobile cell phones?

I bought an HTC HD7 from my neighbor, the reason he sold it was because he didn't pay his last bill and he couldn't connect the phone to any other service. I bought the phone on Saturday and i'm not quite sure when his last bill was due but the phone can still sometimes get in coming calls. My question is, can i connect it now as it is? Or will i have to wait till the service is fully disconnected? Also, does anyone know if i can get the monthly plan rather then getting a contract with that phone? thanks for taking the time to read this, and i hope you guys can help =]
Also I currently am not a T mobile costumer nor do i have any other type of cellphone service.

Firstly - you can connect it now as it is! But I can't say, if you probably will cause any more money for him (on his last bill). But after it is disconnected, you can use it with all the apps he (already) had installed on it! It's because you are using his WP7 - ID (Hotmail/Live ID).
So -as long as you don't reinstall the OS or you do an hard reset with the phone- you'll still stay in that status! BUT (if disconnected) you can't do any calls with it (except the emergency/help calls). Guess you know that already.
For your other questions: you can buy yourself a "prepaid contract" (those, you can cancel by month) from some provider like "Klarmobil" or "Simyo". Means every call you do - costs: 9Cent with total costs between 9 and 20 Euro per month. Those telephone cards will running in your phone without any problems! I mean - your phone isn't sim-locked (branded).

any more questions - don't hesitate and ask!