The brake fluid in my car was changed on schedule and according to MB procedures. My failure, at least, was not due to maintenance neglect.
Calipers are available from Performance Products and probably from PartsShop on this site. To give you some idea of cost, rear ATE calipers for the 123 model, not TD, (the only Performance Products Catalog I have) are $91.41 each. The seal kits for ATE are $15.05 each. There is no mention that these are rebuilt units and there is no core charge, so my guess is that they are new. I have no idea what remanufactured units may cost.
Good luck with your project.