Bad news his 81 300SD is a 126. The Mercedes Classic center does not sell a CD Service Manual set for his year and model.
I think the best he can do would be at Mercedesmanuals-Price: $54.00,Sale Price: $47.00,You Save $7.00! (there is also a shipping charge.)
I think they are also giving you $5 to sign up for their Email.
Here is the manual:
http://www.mercedesmanuals.com/Mercedes_Benz_W_126_Manual_p/w126.htm
This seems expensive at first but the manual easily saves you more money than the origional cost after you do a few repair jobs.
I bought my 300D (123) CD Manual set from Mercedes Manuals for a total cost of $53 only to find out on this forum I could have gotten it from the Mercedes classic center for $27. I do not know why Mercedes does not sell the 126 manual.