Not sure about the rear seat swap, from my understanding the transmission tunnel had to be widened a tad in order to accommodate the stronger driveline.
The seats are Recaro's for all 4 seats, so that is a bonus. I suppose a custom interior shop should be able to make something work? The car will fit the family role just fine if you can solve the seating issue?
The car is fine in winter driving as long as ASR is functioning, I think tires will have the most impact on driver impression in Winter conditions so a winter set of tires/wheels is the best option but overall the car is good.
This also depends on your driving style? From your post it appears that you are in Europe so I imagine that high speed driving will a normal daily event (lucky you), not so in the US.
Does the car have complete maintenance records? The oil by the front of the engine could be from the Power steering pump, have a competent Mercedes mechanic make a thorough buyers inspection.
You should use the search function in the upper right of this page and use terms like 500E and buying, you will get plenty of hits as this has been a popular subject lately.
Good Luck, and BTW, is that US Dollars?