Op, whatever you decide, test drive the exact car (not an equivalent model).
Think about your journeys, do you do lots of short ones? Then diesel is better, ISOFIX seats in the back is a must too.
I have driven vw, mercs and bmw. Hands down I would always go for merc. I would go to a showroom and check out the various models that suit your needs. I've owned 2 sports c classes and they were amazing cars but small boot and low to get kids in and out of, plus not very roomy inside I would get the new b class or the like.
Go for low mileage, going thru a main dealer means you get their warranty (anything under 3 has warranty til it's 3 anyway). Newer cars have low or no tax to pay, either.
Also, there are SO many versions of the same car, so look at their data and work out what is best for you, nippy and juicy or economical. Big boots, heated seats and phone connection are the most useful gadgets, IMHO.
Good luck and hope you get what you need.
I would get a brand new, top of the range Lexus hybrid RS if money were no object.