If I may - I had a similar situation in my car - the PO (one of them) enlarged the radio slot to accomodate bigger radio. There was also an extra hole in the dash for something that looked like a socket for earphones with large jack plug. Obviously to me: if you do not plan to use a larger, modern radio and rather go back to original type of radios that were fitted, the chassis work time is probably the best time to bring the slot size to original dimensions. Doing it later requires dismantling of the whole dash, mess, painting again etc. So I would make really sure that all the holes you have in the dash are original and those that are not original - better make sure you decide if you want to keep them. It is not like in 190SL - where the dash comes off. But in 190SL it is a whole different story as dash configuration was changed significantly over years. Here it is easier.
Instruments rebuild - as advised above - better do it even if they seem to be ok now. I leraned it the hard way - both the tacho and speedo I had to pull out ahd get fixed after the car was done. First tacho and then a couple of months later the speedo, that requires taking out tacho
. And the a couple of monthe later the fuel gauge
. Someone changed the fuel level indication system in my 280SL. I replaced the sender (easy) and found out I had to replace the gauge as well (not so easy).
Another thing, obvious to some, to others not: bulbs. Replace all of them when you have instruments out. Lighting, reserve, charging, brakes/door - all of them. Working/not working -
does not matter. Or change to LED. There are various schools. I am a bulb fan - I like this dimmed yellowish light, it creates mood...
Access to instruments was a nightmare (there are many schools from wich side to get there), now I can do it in 15min with my eyes closed
. But the learning was hard - you do not want to go through it.
I am attaching picture of my dash with bigger radio slot and an additional hole (a little quiz - which one is it?)
You may see the hole down left from the instrument cluster - this is the one that you probably (probably) should not have in 230SL as it is for the hazard lights switch (but not sure, better check it).
My car was ordered without radio. I now have Europa. But I still need to go figure out how to fully eliminate statics.