Welcome to the club. Beautiful car and nice Euro lights.
Engine oil: 10/40, and it's fairly fresh, low miles.
Use Diesel oil because regular oil does not contain ZDDP which protects rotating parts from wear. Changed by EPA order to protect catalytic converters.
Transmission: Needs changing
Uses ATF, the fillup plug is on the front passenger side towards the front of the trans and requires a 14 mm hex available from several specialty sellers. The drain is on the bottom of the trans.
Coolant: new, replaced after draining the system for heater box rebuild.
Get new hoses including the short one between the water pump and the thermostat housing.
Brake fluid: worth replacing, it's old.
Always use new fresh fluid. Get a Motive Power pressure bleeder. Use only flare nut wrenches. Check the rear slave cylinders for leakage. Repair kits are available.
Differential: never checked.
Check with the axle tubes level. Uses the same 14mm hex tool used for the trans.
Power steering fluid: stays full, doesn't leak, not sure what is there.
Drain and refresh with ATF.
Chassis lube: should be checked.
Get a grease gun and a flexible hose. Use quality grease. If you cannot get grease in the zerk fitting, use a heat gun to liquify the old grease and pump in the new. There are many fittings, all front suspension joints, front drive shaft and maybe the slip joint (Your car may have the improved drive shaft with no slip joint.). There are 2 on the rear axle hinge joint.
Be aware there are 8 sealing sleeves for the hinges on the front suspension upper and lower A arms. They must be there and not broken or damaged. They are used to hold grease and protect the joint. Part number A120 333 01 30. Replacement is a pain and it would make sense to replace these if the front suspension is overhauled.
Air filters: should be checked.
One filter and available.
Fuel filters: should be replaced.
One fuel filter between the pipe on the chassis and the fuel pump.
Oil filters are common among several models, get from the dealer.
Brake rotors and pars are available, rear drums are expensive.
There is a screen in the tank mounted from the bottom in the middle. It's got a hex hole and is plastic. They are still available from Mercedes. Remove the tank and get it cleaned.
Service manual CD's are available from MB. Paper ones are available on EBAY.
The W113 SL site has many good pages on service, these two cars are similar. Welcome - Mercedes Benz 230SL, 250SL, 280SL Pagoda Group
. Register and use the common info in the tech manual.
I hope this helps.