Not to sure about 320d s but look at the usual things, like it having full service history mainly BM, low miles, low owners, check the car for wear and tear ie seats and steering wheel are always a good sign, check body panels are straight and no signs of a shunt, check engine bay for signs of leaks, also undo oil cap and look to see if it has cream under it ( sign of a head gasket) once running check exhaust for blue smoke etc and if can take for a nice long test drive to see how it feels but to be honest if it has been serviced regulary not a lot goes wrong with BMs, also do a google search for common problems with BMW 320d and have a look on ebay, they have classifeids and i have just got my 7 from there and it is absolutely mint.....