Steel wheels should be easy to find or even make. I suggest making them so that you can go a bit wider or sourcing them from an S class or another SL. The W123 / W114 / 5 factory steel wheels will be skinny
The SL always had the 6.5j wheels . However the prices for these is ridiculous atm . That is why I am considering the 6j as they are more plentiful & therefore much much cheaper ,