I am thinking of looking at a 1978 300d. Asking price is $1,000. It has 190,000 miles on it. I used to own a 1981 300d. Would this car be a lot slower? Does it get about the same mpg? Anything special I should be looking out for (other than rust and smoke)?

1) slower if non turbo vs turbocharged
2) the Achillies heal on that one is that miserable Automatic Climate Control. The servo has either been changed out with an aluminum rebuilts model, (which is a real plus) or it is or will soon leak coolant all over and will fail to operate and you will get very cold in the car this winter. There are some other changes that can be made on that part, and some, the Euro models use a manual control for the heat and AC, that would be a bigger plus.
Other than that I have had my 78 300D for ten years and 115K miles with no problems. Just a slow smoky old car. I like it.