The piston can be squeezed back in with a slip joint, aka "water pump" pliers, a fairly large pair.
No need to replace the sensors if the pad warning light isn't on AND you can remove it w/o busting it.
The rotors can be reused IF they aren't worn below the SERVICE spec, not the min spec stamped into the disc. For reference the service spec is normally about .3mm thicker than the min. spec., you should be able to find a source for the service spec somewhere online. The service spec is the spec at which you should still be able to get through another set of pads w/o dropping under the min spec on the rotors. Also for reference, most of the time with a new rotor you can replace the pads once w/o having to replace the discs, in other words you can go through 2 sets of pads on a new pair of discs.
Gilly

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