Welcome to the forum, looks like you got a nice low mileage 300CE there!

Spark plugs: gap to 0.032", do not use platinum plugs, they do not run well in these era of engines... go with Bosch or NGK non-resistor plugs

As far as cap, rotor and wires, I would go ahead and do them due to the age of these parts. ceramic becomes brittle, wires start to arc, etc. However, if you replace these parts and it makes no difference in performance or economy, just keep them as spares or put the old ones back on and keep the new parts as spares.

I think the 300E/300CE tank capacity is 18.5 gallons,
so 300 miles / 18.5 gal = 16.21 MPG with an actual figure a bit higher since you are not completely draining the tank every time... seems a bit low, most people seem to get about 20 to 25 MPG with their cars...

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2006 BMW M5 "Heidi" @ 109,000 miles
2005 MBZ C55 AMG "Lorelai" @ 165,000 miles
1991 MBZ 300E "Benzachino II" @ 165,000 miles
1990 MBZ 500SL "Shoshanna" @ 118,000 miles

(On the hunt for a good used M103 engine as of 6/10/23, PM me if you have one to sell!)