Nice looking 280SE.
Here is what you need to do.
Change all the fluids. You need to use engine oil in the 20W-50 range. Modern oil has no ZDDP which is used to help protect the engine and the catalytic converter. Use diesel oil as it has ZDDP or Castrol racing oil has this additive. Check the side of the bottle. The transmission fluid is ATF and the fluid clutch, aka torque converter, has a drain plug. The differential fluid has to be changed with the axles horizontal.
Greasing the suspension is very important. All the joints in the front suspension need grease. If this has not been done in a while, you will need to use a heat gun to liquify the congealed grease. Use a rig to lean off the zerk fittings and grease the joints. There are also a joint or two on the driveshaft.
There are two joints on the swing axle joint of the rear axle. Check your owners manual.
Check the front suspension lateral support rod that goes from the center of the front crossmember to the chassis. There is also a steering damper there. Make sure it exists and is not broken.
Filters are available from the dealer and are easy to replace but are not a spin on. Even today, MB uses a replaceable element.
Air filters are also available. It's a big filter, undo the three clamps and replace it.
There is a power steering filter in the reservoir. Undo the top and remove the plate. The filter is under it.
The engine uses mechanical fuel injection. Do not make any adjustments to the linkage. No not adjust the two stops on the venturi or injection pump. DO not adjust the knob on the back of the injection pump whit the engine running. This adjusts the idle mixture along with an air bypass screw on the intake manifold.
There is a small air filter on the injection pump used for warmup. It screws into the engine side of the warm up regulator. Look at the rear of the pump and you will see a cylinder with two water hoses.
Get an account for the EPC, electronic parts catalog. It has all the part numbers as well as the assembly view drawings for the car.
You will need a credit card that won't be charged, ie it's free. You will need the firts part of the chassis number, aka vin. It stars with 108 and has a 018, ie 108.018. Look at the chassis number and it in the form 108.018-12-nnnnn. The 12 means: 1 - left hand drive, 2 - automatic. The nnnnnn is the chassis sequence number.
The SL113 forum has a lot of good info because these cars use many common parts.
Pagoda SL Group - Index
Parts can be ordered from the dealer or from the classic center.
Mercedes-Benz Classic Center | Mercedes-Benz