Overfilling can blow seals, it's never a good thing for an automatic transmission to be overfull. If it has not been changed lately, drain the transmission AND torque converter and replace the filter. Then refill it very carefully. If you just changed it, carefully pull the plug and drain a little out, recheck, drain until the oil level is correct.
After getting the level correct, don't be surprised if you still have a leak, there could be a blown seal.
As far as restoring this car, I would suggest posting the exact engine in your car. The pro's on this site can tell you about the expense of rebuilding your drivetrain, and tell you their experiences with that particular engine.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in