A 300Tdi auto will most likely be an EDC (electronic diesel control) fly by wire engined vehicle, be warned, EDC parts are no longer available and have not been easily found as good second hand spares of late, converting to manual throttle pump is another cost on top of the vehicle itself so bare that in mind when it comes to how it runs when you test drive it and come to make an offer should it suit you.
Auto box fluid colour and smell should be used as an indicator as to the life and service history of the truck, brown/black with a burned smell to it is an obvious hint that it has been lacking in servicing and had a hard life, could run on for a long time still after service and filter changes though.
Remember the newest you will find will be some 20 years old now!!!
All the usual suspects will be needing attention, suspension bushings, springs and shocks, steering ball joints are all service items and unless they've already been replaced at some point will all need looked over carefully.