The plastic clips are attached to the headliner (permanantly? - i dunno, maybe they're screwed or glued on) and they are maybe 4-6 inches from the front of the sunroof (i'm not too sure on this, its been awhile since i removed mine). Once you've gotten the clips out there are no rails on the headliner to be broken or damaged, you just have to be careful not to damage the headliner in the rest of the car. That is why I recommend that you pull the headliner out from the top. Hold on to it and slide the sun roof all the way open, then you should be able to take the headliner out through the top of the car without bending it too much.

I didn't mention this before, but the work I did was on an 86 190e. The 300e from that era may have a different headliner with more clips. I would just start along one end and gently pry the headliner away from the sunroof with a screw driver.