What kind of damage may occur if timing belt fails or no damage at all and engine just stop working?

I just bought a Hold Jackroo 1996 with mileage of approx 222000km. I have no previous record re timing belt, eg. when it was changed last time?. My mechanic checked timing belt and said it is still very tight and in good working condition so he hesitated to replace it. My question is if timing belt is due to be replaced and is not done, and timing belt dose brake while the engine is running, then what kind of damage the timing belt failurs may cause to the engine? Or no damage at all and engine just stop working? A further question is what kind of check can be done and how often as a precaution re possible coming timing belt failure so it can be replaced in time before timing belt failurs occure? Thanks for your help.