It's certainly worth a look, but repairs to Audis can be expensive so you'll need to check its service history if it has one and try to find out what sort of a life it's had.

With any 13-year-old car, condition is more imnportant than make or model, so be prepared to cast the net wider. If you need that size of car, the advantage of a Mondeo or another Vectra is that there are more around to choose from. Also see if there are any sound Avenses, Primeras or Accords about.

At that age a relaible car is key. If it's nots broken then i would keep it. To answer your question the motorway i guess the Vectra might use less fuel while the Audi.