Exactly, the billboard shader is already in there, so you would need to make non-moving objects and draw them through that shader, then place them. Make sure to check their depth values and such so that they appear on the same height as the vehicles :)
As for 3D buildings, you could create a vertex buffer for them, simple cubes stacked or something, and give them a texture. Look up DragoniteSpam tutorials on youtube, it will help a bunch!