To answer most of TS concerns: No bullshit.
1.) Can you afford with 2k salary with W124?
No - if you have high fixed commitments such as house, loans, insurance etc
Yes - If you can set aside RM 800 monthly for your baby W124 (Inclusive of Fuel and repairs)
2.) Cost of maintaining W124 fleet
Reasonable. Compared with sending Proton into SC, repairing W124 is still cheaper.
3.) Common problem with that car (AKA manufacturer defects, not wear and tear)
None really.
Power window failure? No
Engine over heating? No
Gearbox related? No
4.) Common maintainable parts
Engine mounting, 2 pieces if im not mistaken. Will cost less than RM500 if you know where to get. Will last more than 2 years under normal road conditions.
Engine - Standard engine maintenance. Change your oil on time. Recommended to use Semi Syn.
Check for is oil leak or white smoke or sludge build up. Make sure when you test it, it is smooth.
Gearbox - Standard. Change it on time.
Check for oil leak and gear not engaging.
Break Pad & Disc - Normal wear and tear.
Use original pad if you want your breaking to be quiet.
OEM is alright, might sound like lorry breaking.
Ori pads will cost about RM500-600 per car
OEM pads will cost around RM300+ per car
Suspension System - Rarely needed to change. Will most probably last the entire life of the car.
Check if there is any squicky sound and turning is smooth.
Radiator - Use only original Mercedes coolant. Wont cost much, about RM30+ per bottle. Remember to check water level. Don't trust your sensor too much, old car's sensor isn't reliable. If possible, change the radiator water tank sensor, cost less than 100.
Check for rust and leakage. Check the tubes are still elastic and no cracks.
Timing Belt - Standard. Use bosch.
Check for cracks or any wear and tear sign. Change it immediately.
Power steering - Just make sure that there are sufficient lubricants.
Check for red oil substance on the floor. If there are, chances are from power steering leakage.
Note: W124 might be more prone to power steering oil leakage due to the heat close to engine bay.
Alternator & Battery - Voltage should be between 13.5 to 14v during idle.
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Things to check before you buy:
A.) All of the above
B.) Rust - very important
(This is not MCQ)

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Expensive repairs: (Variable, depending on how serious, but i will give you the worst case scenario)
Gearbox rebuild - Maybe around RM2k
Engine full overhaul - Maybe around RM4k
ECU? - Cheap, buy from chop shop. Very rarely spoil.
Suspension system - About RM2k
Sunroof - Don't even bother to repair it. Will kill you. Just sealed with silicon at the edges.
Just my 1 cent.