Basic question: How much to tighten crush washer?
 

I am going to swap out the oil pressure sensor on my 1991 300CE. It attaches with a copper crush washer. How much do you tighten this? Is it similar to an oil filter, finger tight then 3/4 more turn? Or like a sparkplug and washer? Just tighten until you feel it crush?

NO !
 

Factory Torque Specification for the Oil Pressure Sensor is...20 Nm.
['90-93' 300CE ]
(That should "Crush" the NEW washer properly.)

I do have typical torque wrenches but I don't see how I can get them on the tightening nut that is on the body of the sensor.

Crow's Foot Wrench(On the end of TorqueWrench)
 

I KNOW this is not a "Space Enhanced" task.
[You might have to use the OLD Beam type Wrench]

EDIT! [Ignore my previous suggestions]

(Dagnabitt!)

The Fuel Pre-Heater Heat Exchanger's Metal Fuel lines WRAP around the Oil Filter Housing
ON TOP OF THE Oil Pressure Sensor making access by normal
humans Nigh Impossible!
(I can "SEE" the German Engineer Laughing as he tells his grandchildren how
they eliminated another DIY avenue.)

I'd remove the Fuel Pre-Heater's H.E. and CAREFULLY cut back the Metal lines
(The difference to be made up of Rubber/Viton Lines.)
[Heck,if I can get one of those Mini-Tubing cutters in there,that'll do the job
and allow the H.E. to stay in place...It Looks like a FUN job to R+R also.]

If you do remove the H.E. from the head a new "Sealing" o-ring would be a
good idea. Pn 012 997 51 48

call out # 233 is the H.E.
#245 is the Seal Ring.

For the oil pressure sensor/drain plugs/etc I usually tighten by hand until it stops, then maybe another 1/3 to 1/2 turn to snug it.