I would not just replace the valve seals on your 86 300e . These cars had problems with guides also , I wouldnt worry about rings unless you have severly overheated the car. Check your front timing cover (lower section ) to see if it is leaking oil. If it is have it done while the head is off (it will be cheaper then) Make sure to use permatex (the gray stuff) I average about 15-20 cover reseals a year and none come back leaking. As for transmission leak have someone put it on a rack and clean transmission dry . Run car in air and find where leak is from . Sometimes it will look like pan gasket and end up being something like the control pressure pin seal. Seen that alot.
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck