The sludge issue was limited to the Passats and A4s with the 1.8T. The same engine in the Golf/Jetta had a larger oil pan do to the different mounting style, which eliminated most of the sludge complaints. Many people did not use the correct synthetic oil which greatly contributed to the sludge problem.
6-7.5k is a reasonable price for the car. Even though it has 50k, the car is around eight years old and should receive a full timing belt job (timing belt, tensioner, water pump, thermostat, drive belts, rollers), which will run you around $1000. Tensioners are common failure items on the 1.8T well before the 100k mark, so a timing belt job is a must as tensioner failure will result in valve damage in this application.