Refine STL into STEP

I will provide a nearly-finished STL of a part that now needs to move into the machining phase. The geometry is essentially complete; what I need is a clean, watertight STEP model that a CNC shop can load straight into its CAM workflow... (Budget: $30 - $250 USD, Jobs: 3D CAD, 3D Design, 3D Modelling, 3D Rendering, CAD / SolidWorks, CAD/CAM, Fusion 360, Solidworks)