400E brakes (at least the late 294x25 versions) won't fit early 124's with the M103 engine (or any diesel 124). The 400E rotor is 6mm deeper, requiring either the late lower control arms, OR using the 400E's R129 knuckle/hub. The rear brakes are a bolt-on, other than needing to cut the splash shield. Full details, with part numbers etc, are in my brake spreadsheet:

http://www.meimann.com/images/mercedes/W124_brakes/124_brakes.pdf

http://www.meimann.com/images/mercedes/W124_brakes/

__________________
Check out my website photos, documents, and movies!