Timing belts are supposed to be replaced at 100,000 kms (or shortly before). Did those guys actually take the cover off to inspect the timing belt?

Is the idler arm you are describing the tensioner of the timing belt or is this the idler arm that is part of the steering system?

A 1995 Pajero with 40K on it sounds pretty dubious to me.

You can have the calibration done before or after the timing belt change. That is if your really need to have it done in the first place. Have it checked muna before proceeding with the operation. An unecessary calibration is not really cheap.