Iterative STL Design Modifications

I have several existing STL files that I will email over and I need them re-worked in stages. The main focus is reshaping the current geometry: adding or deleting features, tweaking overall dimensions so the parts fit together better, and then delivering clean, print-ready STLs... (Budget: $70 - $75 AUD, Jobs: 3D CAD, 3D Modelling, 3D Print Design, 3D Rendering, Blender, CAD/CAM, Fusion 360, Solidworks)