Hello OP. There are a couple of people here with Mercedes diesels. We have a 2003 6 cylinder version with 420000 Miles.. no smoke, no leaks, no oil consumption. This was a transit bus and so had a hard working life.
It is electronic controlled so it would be good if you know how to use a multimeter. Any kind of trouble shooting by professionals get expensive real quick.
If this is a local bus then they, or their diesel mechanic, might be willing to help you out if you have trouble later in life.
Not sure where you are but rust would be more of a concern to me then what engine assuming it is running good now. The 4 cylinder indeed might give you good MPG with a 6 window bus. Our 6 has 300 HP so your 4 should be in the 200HP range.
The only part I needed was a fuel pump and that was just available on ebay. Not expensive either.
A used engine runs between 1000 if you are not in a hurry and 3000 if you are not looking around so in case you have real problems like valves , camshafts then price or availability does not matter because you would better get a whole engine.
Whatever engine, buying it from the fleet directly and locally would be a big plus for me.
I have been told by our citie transportation manager that their Gillig had 4 cylinder 4bt engines in city buses but they were for city use. The 6 cylinders were for longer distances. He felt that the 4 cyl were a mistake.
This being computer controlled saved my but when we developed a coolant leak in a hose and the alarm went of when the expansion tank got low. No harm done but it is mind boggling that not all vehicles have that feature.
If you are planning on driving short distances all the time with out having the engine heat up fully then pay more attention to the emission stuff.
Our bus does not have it and had a diesel coolant heater in it. We bought ours directly from the fleet and they told me that they always used it. I took it out because I needed the space and never had a problem but that is just one data point.
Doxylover has a Mercedes engine as well. You see these engines everywhere so parts must be available or may be they are so good that they do not break
. Ask the bus barn
why they are selling it.
What are they asking?
If you buying it directly from the fleet and still can see it in action