Custom Shaders Patch highly recommended. It will work without, but you will not get the night lighting, GrassFX etc.
RainFX is currently only available from Ilja Jusupov aka x4ab's Patreon.
To install, copy the "content" and "extension" folder to the root of your "assettocorsa" folder...
I made this track off CAD files and BTB and 3d Max a few years ago.
For reason I am not sure of, it is located in the existing QR folder as National Course
It is very accurate as I had full access to the track for a day to take photos etc.
It is dated and the new version will be done soon...