I just looked a 1992 300E 2.6 for a friend of mine. I found two things wrong with it that are significant.
1-freon low and got a hit with sniffer in drain of evap case.
2-head leaking oil at block and oil in coolant.

I'm concerned about the condition of head, i.e. could it be warped? The car ran smooth and did not overheat when I test drove it.

I personally deal with head gasket leaks right away, when there is only a little almost beige colored liquid leaking between the head and the block. I'm wondering if he should just pass on a car with a head gasket leak that is this bad.