the 500E, 500SL and 600SL brakes are, indeed, interchangable. however, the 400E is NOT interchangable with the others.
The upgrade from 300E to 400E is restricted to the calipers themselves, and possibly the booster and master cylinder.
However, if you wish to upgrade the calipers on your 300E you're better off going with 400E/300CE calipers. You'll never need 500E/SL calipers on your 300E unless you track the car.
'94 W124.036 249/040 leder; 8.25x17 EvoIIs
'93 W124.036 199/040 leder; 8.25x17 EvoIIs, up in flames...LITERALLY!
'93 W124.036 481/040 leder; euro delivery; 8.25x17 EvoIIs
'88 R107.048 441/409 leder; Euro lights
'87 W201.034 199/040 leder; Euro lights; EvoII brakes; 8x16 EvoIs - soon: 500E rear brakes
'70 R113.044 050/526; factory alloys; Euro lights