Now for the bad news...
Parts 5 and 8 are official Mercedes replacements for what is in fact a single piece of steel.
These aren't really meant to be used as cover pieces - though having seen some rusty old W123s over here I'm sure anything could happen!
If you have the dreaded rot along the side lengths of the car then you'll probably need to get some outer sill pieces (sills = rockers = longerons!).
Dealer price for 5 and 8 is 550 euros per piece.
I've just got a klokkerholm floor piece for my W123 (left hand side) for 130 euros...
My W123 saloon / sedan old accident repair (and a bit of rust repair) thread
Most people cut panels and seam weld them to fit.
I'm playing about and want to try and replicate factory spot welds - this isn't the fastest way of fixing up a car... there are very few (if any) commercial repairers who would consider doing this amount of work on a W123.