OEM MB wheels have several stampings on the back. The part number is something like 116 123 45 67, the size is something like 14J65H2 ET25. The size stamping is what's important to you. The first two digits are the wheel diameter. I believe all steel wheels since the mid 60s are 14". The numbers after the J are the wheel width. You should be okay with anything from 5.5" to 6.5" but check the offset... which is given by the numbers after the ET and measured in millimeters. Ideally you want the same size as you have now. A half inch either way should not make a difference in fitment as far as common steel wheels go as long as the offset is less than 25mm.

Wheels that fit a 107, 108, 109, 114, 115 or 116 will have the same 112mm bolt pattern as your 123. I don't know if older wheels will fit.

Don't pay too much for steel wheels. If you check ebay and the classifieds section of this forum, you'll see that a set of alloy wheels can go for around $100.

Sixto
91 300SE
81 300SD