Mercedes: 1991 Mercedes 350SD, mercedes benz dealer, oily surfaces

Question
Today I test-drove a 1991 Mercedes 350SD with 89K miles. I didn't buy it. The body is in _great_ shape, but there are a couple of problems that I am worried about. I wonder if it will be expensive to fix them and if I should just forget about this car.

1. The engine runs smoothly, but when you try to accelerate hard the engine has reduced power and surges every second or so. This happens at any speed. What is going on? I am worried about the turbo and injection pump. How do I know if the turbo and injection pump are working properly?
2. I noticed diesel fuel on the injection pump and below it. The engine looked like it had recently been pressure washed, and that dirt had stuck to oily surfaces. Do injection pumps leak? What does this mean? How much can it cost to fix?
3. When going over pumps there is a loud creeking sound that seems to come from the back door panels. Where would this be coming from? Is it easy to fix?

Answer
Best possible answer:
Take it to your local Mercedes Benz dealer for a PPI = Pre Purchase Inspection.
They have a much better chance of catching the costly issues.
The PPI cost is far less than buying a money pit.
If the seller will not allow a dealer PPI, "RUN" away from the car.