Have you taken a look at what the stairs are made of? Plywood, OSB, Paricle board etc.
If the stairs are plywood and you can strip any paint and sand the treads, stain to the color of your choice and finish with two or three coats of satin finish polyurethane. Make sure you light sand between coats. I have just completed this project and the stairs look great. Replacing the actual stairs is something to seriously reconsider unless this house is not occupied and you have the time, patience, skill and$$.
Good luck