Hi there,
Jeremy Bullock made youtube tutorial about FPS controller. There is topics about ladders, slope and gravity, stairs.
https://youtube.com/watch?v=Etpq-d5af6M&index=1&list=PLTZoMpB5Z4aD-rCpluXsQjkGYgUGUZNIV
About the stairs solution and getting the raycast avoid colliding with objects that are not stairs, there is safer solution using collision masks and layer that can be setup for stairs and that raycast, so raycast only check stairs collisions.