Basic model requirements:
png or dds textures, in a folder named "texture" which is in the same folder as your fbx file. (that is, the "track" folder contains track.fbx and texture\). dds is preferred, but png can be automatically converted at runtime.
physical (collider) objects must be named starting with a digit 1-9. Anything else will be render-only.
walls (any collider that hits the car body) need to contain the word 'WALL' in their object name. Otherwise it only interacts with the tires.
track surfaces should contain their type in the object name - defaults are GRASS, ASPHALT, these should never be part of another surface type's name (eg. if you have GRASS, you can't have LONGGRASS)
no single object more than 65536 vertices (same limit applies to faces but you will almost always hit vertices limit first)
And some special object names that need to exist to get practice/qual/race modes to work:
AC_PIT_0 - pit box for the first car
AC_START_0 - starting grid for the first car
AC_TIME_0_L - left side of the start/finish
AC_TIME_0_R - right side of the start/finish
In all cases you can duplicate them and increase the 0 to 1, 2, 3 etc. in order to add more pit boxes/start points/timing lines.