I believe DMF life has a lot to do with how the car has been driven, like a clutch, thrash it and it will wear out. Providing that its OK when you buy it if its driven reasonably gently it should last. I bought a Peugeot 406 at 189k with I believe the original clutch/DMF, it has been showing some symptoms of failure but my normal driving style is very gentle and it has done another 13k since I had it with no noticable deterioration.

I think it is more the very low RPM/high torque scenarios which kill DMFs. With a clutch, the friction plate (should) be solidly coupled to the pressure plate, whereas a DMF allows a degree of angular 'slip' via the springs. This is why diesel engines are a bit stall happy nowadays. Manufacturers tend to prefer to let an engine cut out if the RPMs drop below idle speed, and to restrict torque at very low speeds. This has curbed the number of DMF failures, but makes the engines a bit less willing in stop/start conditions.

The advice is to start the engine with the clutch depressed to increase DMF life.