If you're looking for Chassis & Body Manual, they are available on E-Bay.
https://www.ebay.com/itm/MERCEDES-SHOP-MANUAL-SERVICE-REPAIR-BOOK-116-CHASSIS-BODY-WORKSHOP-450SEL-300SD/233256141956?_trkparms=aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D225640%26meid%3Db94226 ca76024250b6ff7daae6a2b88e%26pid%3D100005%26rk%3D3%26rkt%3D12%26mehot%3Dco%26sd%3D193290817975%26itm %3D233256141956%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv5PairwiseWebWithTitleNsfwFilter&_trksid=p2047675.c100005.m1851
(I know it's for the 420SEL but the chassis will be the same.)
If you need Engine Manuals, the W126 manual should get you very close. The difference might be transmission control.
http://www.peachparts.com/shopforum/general-information/248948-mercedes-service-manual-source-cd-dvd-paper.html

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )