Hi,
I have owned 12 Audi's over the past 25 years, all but 2 being A4 Avant
diesels, I am currently on my 2nd A4 Allroad, a 67 plate which I cannot fault.
all of my cars have been 2ltr, I see the one you are considering is a 3ltr. as
the car has only 23,000 miles I would not be concerned, as that engine is fitted
across the Audi range, if you are buying from a main dealer, you will probably
be offered a 2 year warranty anyway, regarding other things to look out for, in
my opinion there is nothing really, as long as you are happy with the car.
by the way you have extra ground clearance with the Allroad as well, and there
is no a great difference in fuel consumption.
hope that helps
Brian.