Your plan will be more than strong enough for dead weight on a perfectly level/planar basis. Just remember that the floor may not be perfectly level when you build this, and I wouldn't trust shims under 2000lbs + weight. I would put plywood around the base to give it lateral stability as well(unless your sump will be under it, then I would ply 3 sides and angle brace the last. You never know, the New Madrid fault may open up under your living room!!You can find plenty of scrap where new construction houses are going up, leftover from sheeting the roof.