Cool [Q] What sould I do?

My wife handed me down her HTC M8 (Tmobile) 'cause she jemped to LG G3. The phone currently has 1.57.531.7 and it's flashing new update from HTC or Tmobile for a new update 2.22 something.

I want to root the phone and put custom ROM on it. So my question is should I stay on the old firmware and do all that or should I take the update and then start to play around with the phone. Does the new firmware present any trouble if I try to unblock bootloader, root, S-off and put on new ROM?? I'm no stranger to play around with the phone (My current phone is Samsung S4 which has been flashed with many ROMs) just that I haven't followed HTC M8 scene so has no idea the best way to go about it.

Thank guys