Bought the rebuild kit from FCP. To my surprise the kit doesn't come with new piston.
I also replaced both rotors and pads with Zimmerman and Textar pads.

Caliper rebuild process is not too bad.
I used the brake system to push the piston out. It was a mess but doable then I pinched the brake hose with a vice grip to stop fluid leaking out while replacing o-ring and seal.
The piston had a little rust spot, I sanded it smooth using 400 and 1000 grid wet sand paper.

Glad to have my ride back this morning.

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E70 35D
E46 ZHP Coupe
E46 330XI
E83 X3