It HAS to be the SAME brand caliper. You musn't use different brand calipers. If the one on your car is an ATE, then the donor must also be an ATE...not Girling.
Before I sold my 535, I ended up replacing the calipers also. They were somewhere around $45 after core. Not too expensive.
I know it can be done, but rebuilding the caliper with a kit is just a waste of time in the long run.