I wrote quite something that somehow got lost. Not familiar with extensive body work on these large topless unibodys, used to do some bodywork at a Mercedes dealership long ago, so if maybe someone more familiar will chime in.
Necessity of bracing also depends on the strength of the whole body structure like rust issues inside the rocker panels and their inner reinforcements for example and how large the sections you cut will be, if they extend to the inside rocker panel f.e.. I'd also do one piece after another, so the structure isn't weakened more than necessary.
First thing if bracing is needed would be both A-pillars, the B-pillar stumps from one side to the other plus an x-brace for the "door opening" . The back shouldn't be too much of a problem if the rear valance stays in again from experience with smaller cars with superior body structure.
I prefer doing body work if possible and no "collision repair frame" (don't know the proper word in English) or a similar construction with the car standing on its wheels and axles, cause otherwise you already have some bend that could be problematic for later alignment.