Given that you need a large estate, make frequent short journeys but also need to keep running costs down, we would suggest looking at the Skoda Superb Estate. The latest generation version is available as a mild-hybrid petrol or a plug-in hybrid, so with the latter you could make shorter journeys on electric power alone if you can charge at home. Even the previous generation Superb Estate was available with the efficient 1.5 TSI petrol or as a plug-in hybrid too. The Superb is a very comfortable car and should be cheaper to buy and to run than the Mercedes, BMW and Volvo equivalents.