1990 300se question
 

how are these 6 cyl motors?

M103 inline 6
 

You won’t win many drag races... but they are adequate for normal driving.
One of the best ever built for durability.
For instance there is no real interval for replacing the timing chain.

are they the same motors?
 

in the 's' class cars and the 'e' class?

Yes.
http://catalog.eautopartscatalog.com/mercedesshop/sophio/model.jsp?header=header.jsp&footer=&partner=mercedesshop&baseurl=http://catalog.peachparts.com/&year=1990&make=MB&clientid=catalog.mercedesshop&cookieid=1FX0MB1RS1J10ZTXG9

Not what I'd call economical if you run with the top 25% of traffic. My 91 300SE netted 17 city/19 hwy while the S420 gets 16 city/20 hwy. I prefer the car that can keep up with an ES300 when necessary.

That engine is more balanced in a 300E, that body is more balanced with a V8.

Working on that engine is like shooting fish in a barrel.

Sixto
95 S420
87 300SDL

They burn oil, also a little to much car for that engine. If you are going with a gas car just get the biggest engine availible. All gas W126's get poor mileage might as well have a big V8 to enjoy while you are feeding it. :D