+1 keeping the system 7-speed, just replacing worn out stuff. I have my touring bike set up as 7-speed for a few reasons:

1) Stronger chain.
2) I can use my original Sugino GP half-step crank with granny.
3) Friction shifting works just fine with 7-speed. 9-speed is a bit more finicky.
4) 7-speed MTB parts are widely available and cheap.