This information might help - on standard W123 14 inch wheels the largest size tyre that most people say can be fitted is 205 / 75 R14. From what I can work out you should be on the edge of rubbing rubber on metal with a diameter of that size (what ever it is) with a standard suspension set up. If you lower, however, you'll have to search about to see what others have done to make things fit.
There's a tyre size calculator in the sticky at the top of this forum.
EDIT:- The standard sizes / offsets etc are listed in chapters 40-102 and 40-104 in the W123 FSM
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
Don't leave that there - I'll take it to bits!