Hi, Welcome to Just Answer, I am Clayton. I can help you with this.
Alright, lets start with the Mercedes Side.
The most common problem I see with the MBE 4000 are coolant leaks, and fuel leaks. Coolant usually leaks on the coolant rail, it is a metal rail that connects all 6 cylinder heads together, it is all so called the Deareation line. They usually crack, or the crush washers leak.
Some may mistake this leak as a head gasket leak, but if you looked closer at it, you would notice the leak above the head gasket.
Fuel leaks are VERY common, the most common place is a plastic line from the fuel filter to the side of the block. There infact is a recall that replaces the plastic line with a metal one to prevent this common failure.
Fuel leaks are also common on the high pressure fuel lines, if improperly torqued, the nuts will loosen and fuel will leak. If this occurs, the high pressure fuel line, and pass through tube will have to be replaced. If you have to remove the lines, Detroit does not recommend reusing the high pressure lines, it is advised to replace them and the pass through tube.
What is good about the MBE 4000, common tune-up procedures like valve adjustments are very easy. Mercedes does have a flywheel turning tool that I suggest buying for this and other reasons. It should not be very expensive.
I have seen in rare cases that a camshaft wears, if this happens, the engine will have to be removed to remove the camshaft.
EGR coolers can leak, I do not know emission laws in your country, but all MBE 4000's are required to have EGR components in the USA. If yours does not have EGR, consider yourself lucky, because EGR coolers are not cheap on these motors. An indication of this is coolant loss, with no external leaks. White residue or coolant found in the EGR valve. EGR valves are common, usually soot builds up around the flap and causes the flap to stick and will set a fault code and derate the engine. Being a fleet owner, you might be able to remove them, clean all the soot around the flap and put it back to work, otherwise it will need replaced.
Maintance is critical, Fuel and Air filter maintance, and or course oil is critical. That is on most engines.
The turbochargers are rather expensive, a hole in an intake pipe, or dirty air filter can eventually cost you a couple thousand in parts, inspect this close.
Now for the Freightliner side.
Electrical problems can plague you, Freightliner has had wire loom that can murder wiring. Look for the plastic wire loom that is black, with a green stripe. Guarenteed somewhere, the wiring is getting chaffed.
The headlight harness will rub on the bumper and cause a short. Inspect the wiring that goes to the headlight assemblys for rub throughs.
Plastic interior pieces will get old and brittle, very common on all these trucks regardless of year.
A/C will quit working because of leaks, usually the condensor leaks where they are welded. The only repair to this is a new A/C condensor.
If oil leaks on the rear-motor mount, the driver side mount can wear prematurely, an indication is lots of movement in the motor.
If you need more info, let me know.