The cheapest sub-flooring material is strips of 3/4″ plywood that are narrow enough to fit through your access hole. The boards should be long enough to cover a span of at least three joists, depending on what you can manage in the available space. Tongue and groove lumber measuring 1 x 6 inches is also good for the subfloor.

Floor covering might be sheet vinyl, laminate, wood, linoleum, carpeting or vinyl tile. If you have any concerns about the additional weight of the finished room, choose a lighter floor covering material.

Also important to consider is how you will get the material you choose up the stairs and into the finished space.