engine mount(s) broken
 

I have recently purchased a 2004 SF 4 cylinder 2WD with 71k miles.

So first thing I did is replace the timing belt and serpentine belts as well as front struts. not to mention changing of oil and filter, air filter.

In doing this i see I have to replace the top engine mount.

A) Is it advisable to replace all?

B) Is this common and what would cause this.

C) also noticed how extraordinarily expensive these are to replace. any place I can find these for a reasonable price.

D) anything else that should be inspected that is critical.