Is the house over a basement or crawl space that will provide access to plumbing? Is your drain plumbing cast iron or ABS plastic? You can get free estimates, but be ready to answer these questions. Try to find a tradesman recommended by friends or neighbors or better yet, a really good handyman friend (or friend of your dad's).
Your project can be done reasonably as long as there is access under the floor. Be sure in designing (or purchasing prefab) a shower fixture to spend the extra $ for a built-in seat and safety grab bars. Remember that a bath adds substantial value to your house. A loan would be a very strong possibilty to allow you to do this project right with minimal financial impact.