Repair Architectural STL For Printing

I have an STL of an architectural model that should look like a miniature building once printed, but the mesh currently contains holes and other non-manifold errors that stop my slicer from generating clean G-code... (Budget: $10 - $30 USD, Jobs: 3D Animation, 3D CAD, 3D Design, 3D Modelling, 3D Printing, 3D Rendering, 3ds Max, Blender)