From someone who has a 1975 300d I can tell you that chances are that the 75k on the odometer is not correct. The odometer in these cars does not have a 100k mile digit so the car could have 200k or 300k or more on it.

At 75k miles you should have nearly perfect compression. My 115 had 104k when I got it and the compression was nearly 300 psi all round. At that milage virtually nothing in the engine compartment should be worn out. Not unless they never changed the oil.

Take your injectors to a shop and get them popped. They will probably tell you that they are not good in order to sell you new ones, but you could ask to see them tested and ask what is wrong with the spray pattern. If your injectors are miss matched and worn out you can bet that you have a lot more than 75k on the engine.

You may have new GPs but are they all working. I bypassed the GP relay and put a starter button on the dash directly to the GPs. It works like a champ. Hold the button in for a few seconds (I live in S. Texas) and off she goes.

Try the oil in the cylinder trick when you check the compression. If the compression comes up then one, you know your rings are bad, and two, you know that the engine has more than 75k on it.