What tire size, diameter, wheel size/type is the most ideal/efficient? There are the original wheels (14 or 15) on my '83 300SD, would larger (16, 17+) tires/wheels have better performance? What about diameter? Low profile?

Any comments are appreciated.

No such thing as " ideal ".
If you currently have 14 or 15 " wheels, you could go to 16's or 17's.
By adjusting the tiresize, you can maintain the same overall height, i.e. rolling diameter.
Most people switch to larger wheels & low profile tires to increase handling.
In your case, given the fact that you have a 22 year old car ( with a 22 year old suspension design ), I really don't see much benefit to making the switch.
Besides, your ride comfort will suffer with the lower profile tires, and they are fairly expensive to replace.
It's your call.

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2007 C 230 Sport.