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They will fit the GS300 just fine. You will probably need to buy new rotors. I'll see if he still has his stock ones. Maybe I'll just include it in anyways.
As for the 20" rims, it depends on the disk of the wheels.
As of right now yes. I can have it shipped directly to your place, or I can have it shipped to my place, and you come pick it up. I'll probably have the brakes next week.