Not Sure about the difference in bumpers but skirts should be pretty easy to do and give a large (for the labor) benefit.
Before you do the rear belly pan, make sure to read
this thread
Talks about how heat trapped under the belly pan covering the tank can "boil" the gas and overload the Evap system.
Here is an Template Overlay (of a 93 SL1)
Those Hub caps would have been nice.
Looks like you could comfortably go out to 15% template without restricting any rear visibility if you make a kamm/roof spoiler.