I don't think I would want to build it on site because that requires some machinery for a good finish. Also, it's not easy to saw in 300 mm thick timber *shudder* I would probably make it from 225x45 which I would then glue together, makes it a bit easier to handle.
The steps should probably be cut with a table saw or a Festo with a rail and then trim the edge.
Make a full-scale drawing on paper/particle board, and it will probably be easier to tweak it.
/Kent