If large and reliable are priorities, then the obvious estate at that budget is a Honda Accord, though a Toyota Avensis or Mazda6 would also be possibilities. You can get a 2007 diesel one at under £7k.

Comfortable is in the eye (or back) of the beholder, so try one out.

To be honest, I wouldn't worry too much about the number of miles that it has on the clock, as long as it is in good condition.

If the Passat is coming up for eight years old, it won't be worth much in the market, so if it's going well why not keep it until it goes expensively wrong, or your family becomes six, whichever is the sooner!

If no. 6 is imminent, then maybe look for a people carrier such as a Toyota Verso or Mazda 5, or a big Kia if you want something large and cheap. Don't go for a used diesel unless your mileage is very high: many diesels are prone to expensive failure as they get older.

Either way you should be able to get away with keeping the Z4 for when you're driving alone.