If I understand what you are referring to, the rubber spacer is part of the motor mount and cannot be replaced by itself. It is bonded to the metal of the motor mount.
the manufacture reccomends it be replaced in 90,000 miles
The Mustang V6 does not have a timing belt it has a timing chain which normally will last the life of the engine.
If the engine in that Mustang is maintained properly the catalytic convertor should last the lifetime of the vehicle.
It should be replaced every 90,000 miles.
Probably about 150
You do not have to replace the timing belt but you will have to remove it in order the replace the pump. Unless the belt is pretty new I suggest you replace it at the same time you replace the pump. The pump should have been replaced when you replaced the belt at 105,000 miles.
How often you should change your air filter depends on how much you use it. If your air conditioner is not blowing enough air. That is generally a good sign that it needs to be replaced. After you replace it, continue to check it regularly and replace it whenever you think it needs to be replaced
The oxygen sensors that connect to the exhaust system. There should be two, and they should screw into the side of the exhaust.
Replace it every 90,000 miles.
Replace it when it will no longer take or hold a charge.
You replace it when it fails.
The electrical inspector will make this decision as to what circuits should be replaced.