Hello Ulfi,
The standard W123 steel wheels will work fine except that they have no recievers for the small original hub caps used on the 230SL and 250SL. You can order and add these "recievers" to the wheels. You can also use the later "one piece" wheel covers instead, which is not quite original on your car. The aluminum W123 disc wheels are most sought after by W113 owners. They also work fine, save weight but only accept the later "one piece" wheel covers. If recievers are added they will accept all variations of wheel covers or hub caps.
Look for a W108 six cylinder sedan from 1966 through 1970 250S, SE, 280S, SE, SEL (six cylinder engines). They should have original equipment W113 steel wheels which accept both the early small hub caps and the later full wheel covers. The part number is stamped on the wheels #108 400 00 02.
Joe Alexander
Blacklick, Ohio