I am aware of the Gen II 560 SL front brakes on the Gen I 500 SEC

Perhaps my search skills need improving...............
Is there another Cheap Charlie upgrade?
Wheels are 15" but that could be changed.

I have Brembo 996 4 pot radial callipers left over from another project, and making a bracket to the W126 spindle is within my skill level.
Is there a bigger rotor that will fit, or fit with a bit of machine work?

Is there a Wilwood hat with the correct offset, that will fit or can be drilled for the W126 Hub?
I am willing to do a bit of machine work to make it work

Hh:

If your objective is to upgrade the front brakes of a Gen I W126, the first step is to install the calipers and rotors from a Gen II W126. They are a direct bolt-on.
The '86 onward R107 cars used a front suspension and brakes derived from the W124 chassis. The uprights, spindles, hubs, calipers, & rotors differ in design and construction from those of the W126.
Conceivably a 126 rotor could be machined to form a hat for a separate, bolt-on aftermarket rotor, and a caliper bracket created to mount any desired caliper.

The Gen II mod is easy for those with 15" wheels, but is it the best solution for street use, given an acceptable cost?
Is there a noticeable improvement?

If constrained by the wheel, bigger callipers on a quality 300-310mm rotor, is a simple solution, and retains the 15" wheels. Wilwood has a selection of undrilled hats, so with a bit of machine work to fit the hat to the 126 hub, one could piece together a solution. Presumably this is the approach of the big brake kit providers
Any more brake, dictates larger wheels

I have 996 Brembos and Wilwood 298 x 20.6 rotors from another project, so thinking to recycle these for now with a new Wilwood hat, then if more brake is wanted, simply change the rotor and calliper mount at wheel/tire time. Calliper and hat remains the same

Or buy an off the shelf solution from someone that has already figured this out, but half the hobby is figuring it out for myself and my mates.

Gen II W126 rotors are 300 x 28.

I "think" that I have a Cheap Charlie solution for the fronts figured out using MB 332 x 32 rotors, and minimal machine work
Will confirm once I get the parts in hand

Any big brake rotor solutions for the rear?
The handbrake complicates things but it is not unsurmountable

What exactly are you trying to accomplish here besides drastically overcomplicating a brake upgrade? The Gen II brakes are vastly superior to the Gen I setup and are a direct bolt-on upgrade for the Gen I chassis with the exception of requiring a 15" or larger rim. The Gen II setup will stop even the LWB 126 very aggressively.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I am aware of the Gen II mod, and want something better.

I am too. I am even considering making some 4 piston calipers that bolt on if I can not find something that lends itself as a practical alternative.

I now have bought Brembo rear sliding piston axial callipers, with electric handbrake from a Caddy ATS. The electric e-brake negates the need for rear callipers with drum style handbrake, and opens up the rotor options a lot. Ideally would use 320mm OD rotors

Searching for 320mm OD rear rotors now to balance with the Caddy ATS 4 piston Brembo axial front callipers.

Rotor OD F and R is Caddy sizing, so worst case I will use a proportioning valve

Making a bracket to mount the axial callipers is easy
16" min wheels