Hiya,
I have a BRDM-2 as well, and this one is pretty good and cheap: https://www.afvhandbooks.com/afvshop/prod_7327168-BRDM2-Maintenance-Manual.html
Depending on how good your German is, you can also look at getting the East German manuals (Look for the SPW-40P2, that's the East German designation for the BRDM-2).. I also have a Soviet/Russian parts catalog, but that is 580 MB so a bit hard to transfer..
The engine is pretty similar to the GAZ-66 engine, and some GAZ-66 parts will fit such as the water pump and some of the gaskets..