I agree with RC--get a Cayenne Turbo (not the Panamera).  You will love the room for the kids, the trunk for all the stuff you are going to need for the kids, and it is fast as hell--probably faster than your Targa.

I have had many Porsches, primarily 911s.  They are wonderful, but not for a family.  I have two kids (age 15 and 9) and my Cayenne TTS is by far the best Porsche I have ever owned for a family.  Fast, comfortable, easy to load and get into/out of--fantastic finish and problem free.

The Panamera is fantastic, but more of a gentleman's car.  You will go nuts telling your kids to keep it clean, don't scratch the seats, etc.. 

On the Mercedes, check it out carefully.  A very good and trusted friend--a real Merz freak--had so much trouble with her E55 wagon she turned it back in after six months.