On the one hand, the BMW 3 Series is popular with police forces because of its indestructibility and high-mile capability. On the other hand, an older 3 Series (diesel models in particular) will require a fair bit of regular maintenance and yes, this will be costlier than in a mainstream choice. Parts will be a bit pricier than something like a Volkswagen Passat and servicing (especially at a main dealer) could be more expensive. Insurance will depend on factors like your age and driving experience. If you want a practical estate car that'll be cheap to run, look at something like a Toyota Avensis or Honda Accord.