Wife has an A4 1.9 TDI 2003 (75,000 miles)
I drive A6 2.5 TDI 2002 (129,000 miles)

Looking at updating the A6 - Budget Trade value of A6 + 8K -10K max.

As we have two kids need space for long journeys ( London - Scotland 4 times a year.) Also I drive 70 miles a day mostly motorway.

Looking at perhaps 03/04 plate A8 do not want a long wheel base as a bit too big.

Please let me have the good bad etc of the different engine sizes and options.
What are the real things to watch out for on service history and nightmare things to go wrong.