inherited 1975 240D - lots of questions

Unfortunately, my grandfather is not able to explain a number of things to me about his 1975 240D. He bought it new and it has 82000 miles on it. I think he has had it serviced regularly by a MB dealership. I do not have anything but a parts-type manual for reference. Where do I get a good owners-type manual? Why doesn't the fan work for heat but does work when the air conditioner is on? When running, is the oil pressure supposed to stay at the peg at 45 psi? Is the water temp supposed to hover around 175? What is the little knob just to the left of the key -- if I turn it the idle changes -- It turns about 180 degrees. I seem to recall the passenger door locks being able to be controlled from the driver door, but that apparently no longer works. Any ideas? How much is too much in terms of engine knocking? How much is too much in terms of exhaust smoke? Why is it more difficult to (re)start after its been running for awhile than it is after it has sat overnight? I would certainly appreciate any ideas you may have.