1, ETS= Electronic Traction System , a control module monitoring / adjust wheel speeds and manages the best possible traction and avoid slip. It works together with the ABS.
The car has to see an SDS (OE MB) diagnostic computer and retrieve diagnostic trouble codes and live data of the system to find out what causing the problem.
2, You can buy the part, do the job yourself: hard to tell without knowing what's the exact problem.
3, It's a complex system with many parts, like sensors, pump, control modules, wiring harness incl. network cables.
You can google: How MB ETS works? I bet you will find lots of useful info.
Zee