Should the water pump be changed at the same time? What is the expected cost in the Baltimore Washington Metro? How urgent is this repair at 120,000 miles
It is not necessary to replace the water pump when changing the serpentine belt.
It is considered wise to replace the water pump as part of a timing belt job.
But as far as I know, the Focus does NOT have a timing belt. It has a chain.
On your car, I’d replace the water pump under these condition:
Leaking
Noisey bearing
Bearing has play
Not flowing enough coolant (eroded/separated impeller)
If you don’t have any of those symptoms, leave it in place
Older Focuses definitely used a timing belt, and the water pump and tensioner pulleys should be replaced with it as they both run off the timing belt. If you have an older Focus with the Zetec DOHC engine, I would consider this to be an urgent concern at 120k miles since the Zetec motor is definitely an interference engine. This means if the timing belt fails, the pistons will collide with the valves and you will be needing a couple thousand dollars worth of repairs to get the car running again.
Replacing the water pump as part of a serpentine belt change, on the other hand, is not a typical service upgrade; however, a new serpentine belt should accompany a new water pump since the belt is already off the car, unless the belt is fairly new.