What he said

...but I do question the bit about the body being galvanised , or at least galvanised
properly . I know its difficult to mess galvanising up and MB have stated that all cars after a certain date were galvanised but that does not stop in my experience them rusting , possibly due to poor preparation before the process starts.
W/S 203's are very well known for rusting on the rear arch , so much so that pre formed weld in panels are available , but it does seem a bit hit and miss (bit like W204 subframes) . My own 156K C55 is showing signs of decay in this area in my opinion mainly due to the wheel arch lip not being cleaned out often enough ...but which 'normal' owner does that ? the estate tailgate is also problematic . The rot starts well hidden behind the plastic number plate plate , once bubbling is spotted where the plastic part meets the metal it is already far gone .
Again (IMHO) it's due to bad design and poor choice of materials . Basically salty road water gets between the metal and the plastic , its held there by the soft foam like gasket on the 2 number plate lamps , rubber gaskets might have been a better plan but I digress.
The ones you are looking at will probably be in good condition but these areas should be inspected .
I do virtually all of my own maintenance , the 16 spark plugs are fun to do as is the Crank position sensor

. It is a very 'analogue' car and in my opinion better for it , especially if you know your way around a toolbox.
I have had mine for around 9 years now , it lives outside and is pretty much a daily driver.
Personally I prefer the look and experience of the Estate , OP , You say one of the cars is a C55 coupe ? they never made a C55 203 coupe .
A Japanese import will almost definitely be rust free.
This image is of excessive rust on a 203 but this is where you should look to see if there is any paint bubbling here and on the top of the rear arch.
View attachment 172263