To answer your first question, I would say a '88-'92 300E with the 3l M103 would be the best overall choice for a car thats easy and relatively inexpensive to maintain. Gas mileage is not as good as the diesels but still acceptable. The earlier ones are good if the valve seals and head gasket have been done. Look for one that has been maintained and you can't go wrong. The 3L M103 is probably the best six MBZ ever built, I have seen cars with more then 400K.Mine just turned 204K and drives like new.
I love my 400E, but what a difference in prices for some parts and not quite as easy to work on. The highest milage I have seen on a 400E so far was 248K. I'm not sure how much they can do if properly maintained. Perhaps somebody else does.
81 300 SD grey/palomino 168K miles
84 190E 2.3 black/grey 64K miles (wrecked)
85 190E 2.3 maroon/palomino 92K miles
88 300E desert red/palomino 204k miles
(made to look like a '94-95)
92 400E desert taupe/creme beige 120K miles
(converted to 94-95 E420 looks)