You can install the Electric Fuel Pump between the Plastic Primary Filter and where the Fuel from the Tank comes off of the Fender Well.
If you installed the Electric Pump direct to the Seconday Filter you would have to do something about the stock Fuel Supply Pump because the Piston inside would still be moving and it is lubed by the Fuel going through it. With No Fuel going through it you would eventually ruin it.
You might simply ask your local Bosch Fuel Injection Shop how much they would charge you for the Fuel Supply Pump rebuild kit.
Anoter possible issue:
FUEL PRESSURE RELIEF VALVE THREAD/ OVERFLOW VALVE THREAD
Fuel pressure relief valve adjustment - PeachParts Mercedes ShopForum