If you buy a good maintained 190E, the difference in economy would be noticeable.
The 300SE needs more fuel and is more expensive to service in comparison with that 4-cylinder in the 190E.
I don't know how the rules in the states are, but here in the netherlands the difference in assurance money and road tax, for these cars, is very big and that is probably the reason why most people drive in compacted car-models.
The 190E will be fast enough, although it is not as fast as a 300SE.
The top speed of both cars is different (in europe) but the US models are limited at 130mp/h anyway, so that should not make much of a problem...
with a minor 600+k