Sorry, I meant to respond to your other posting too...but better late than never
I owned a 1986 300E 5-speed for 5 years, bought it with 180,000 miles and drove for roughly 70,000 miles before selling it to another member on this forum. The transmission is solid and shouldn't be showing signs of wear with 112,000 miles.
Overall, the W124 is a fantastic chassis and the 5-speed manual really makes it shine. If you have to open it up you may as well rebuild the whole thing. I don't think it makes sense to just fix 1st gear. No idea what costs are involved.