Where is the high side A/C port '88 300SEL?
 

I found the low side:
http://www.fototime.com/33DAA40D09C8AD7/standard.jpg

Looks like it was converted to R134 at some time. Where is the high side fitting? The compressor is turning but the air is warm. The low side pressure at rest is over 100psi.

Either in front of the radiator by the air scoop or on the hose under the air scoop. In your pic it will be out of the frame in the lower left corner.

I found this in front of the radiator:
http://www.fototime.com/FE3AEE7C4CC1FF4/standard.jpg

or this on the compressor:
http://www.fototime.com/9E86799862B0B2D/standard.jpg

Use the one up front, that is the high side. It appears you haven't had a real conversion, somebody just put a low side fitting on and shot some 134a at it. You'll need a high side fitting if you're gonna go 134a.

The fitting on the compressor is another low side port. The "S" and "D" are "suction" (low) and "discharge" (high).

How do these models take to R134? Does that adaptor just unscrew? I could just as well go back to the old stuff. I have half a 30 pound tank of AutoFrost ready to use.

Poorly. Its my opinion that the evaporator is too small relative to the cabin size to effectively cool the car with 134a. The adapters do simply screw onto the original R12 ports.