For street use, get Porterfield R4-S pads, and maybe stainless braided lines. That will make a significant improvement, although the R4-S pads aren't cheap. I'm not sure what, if any, larger brakes will bolt on to a W126. I've heard of installing W126 brakes on to the W123 as an upgrade though.
The Movit setups are $4k+... are your pockets that deep? Might as well get the RENNtech/Brembo setup on eBay right now and fabricate custom brackets for them. Or my Brembo setup (for sale in the Parts section of this forum) and adapt them...
