Fortunately you're driving a Mercedes. As long as the body of the car stays solid and rust free it will almost always be more economical to replace components, even engines and transmissions than it would be to buy a decent car for the same price as your repairs. As mentioned before, these cars aren't designed like the north american cars we all grew up on. Hard driving within reason shouldn't be a problem. Just up the frequency of basic maintanence like oil, tranny fluid, coolant, brake fluid, etc. Even with hard driving the best to get the longest life from your car is to be rigorous with these basics and fix everything that needs to be fixed as soon as you notice it.
1995 E420 - retired
1986 420SEL - retired