Overall, diesel engines are extremely reliable. They make the blocks and components extra strong due to the high compression required to ignite the fuel/air mix in the cylinders. My greatest concern about high mileage diesels (like mine which has 92k on it) is the emission equipment. Parts such as the NOx sensors (I was quoted $1800 to change both) and Urea system scare me a little. I think in the later model years they got it down a little better but the first few years were tough from what I've heard.

Regarding the possible effects of your constant towing, I think that probably help reduce carbon build up in your engine, as the increased load helps the engine burn the fuel more complete reducing carbon build up. I actually think the reason they took the 35d out of the much lighter 3 series was b/c the engine didn't have enough load, which was causing tons of carbon build up. Now they use a four cylinder turbo diesel in the new ones. Towing would have more of a negative effect on your transmission if anything. Have you serviced yours regularly? I guess the wear on your transmission depends on how often you've changed the fluid, how far you tow, and how close you are to the max limit when you tow. I don't have a clue about turbo reliability lol. So I guess to sum it up, it depends! But at least you have some factors to consider.