A6 tdi filled up with diesel

My brother in law wants to sell me his T reg A8 tdi with 80000 miles on the clock because he half filled it with petrol and it hasn`t run right since. When he did this he filled the rest of the tank with diesel and continued to top it up. After a week he used the tank up and filled it completely with diesel. He has now used the full tank of diesel up and it still isnt running right so he has give up on it and opted to go into his companies "company car scheme". He offered me the car for a very silly low price and i am very tempted. I took the car for a drive to day and the symptoms were that it is very sluggish almost like it is in a limp home mode, there is almost no boost available but i know the turbo is fine. Also hard to start when the engine is warm. Any advise on this matter would be helpful I want to buy this car but i am unsure of the damage it may have cause. I do know that when he originally did it there was more diesel than petrol in when he drove off. The car has never cut out and died altoghether its just very sluggish. If i buy the car i plan to change the fuel filter and clean the pump out and fill up with diesel as well as a full service. As i said any help would be great on this matter.