The shoes for the rear are for the parking brake. Change them if they're worn. Ditto for the rear pads and rotors. Generally rears last a lot longer than fronts. No sense in spending the money if you don't have to.
For the fronts you should also replace the pads sensors. They don't come with the pads.
Yes, the brakes are a very easy DIY. This is a very good source for step-by-step instructions:
http://www.pelicanparts.com/techarticles/Boxster_Tech/49-BRAKES-Replacing_Brake_Pads/49-BRAKES-Replacing_Brake_Pads.htm
The book's well worth the $23.95. One job and it's paid for itself.
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