BKN engine - any known problems?


Is time for me to change the car. With two small children the A3 is not enough anymore.
I found an A4 from 2005 with 3.0 tdi engine, quattro and manual gear box. It looks nice and soon I'll take it for a ride to see how it drives. I'm also going to take it to a mechanic to have a look.
I would need your help too. Can anyone tell me what are the common problems of this car and particulary the BKN engine? What should I look for?
Thank you!