Well, unless i missed something not seeing any basement stairs. Depends on if you want a crawl space or not. If you are going with a crawl space no you won't need any fill or a retaining wall. If you don't want a crawl space, and this is how I would do it and wish i did it, I would in fill with sand and pour concrete. Gives you nice solid floor for tile, wood floor and in floor heat. Residential builders don't like it because, all the plumbing and the like have to be right on. Personally wouldn't want to have to mess with a crawl space and full basements are over rated and expensive.