Invar would be great, but it'll be expensive and wonít come in many forms. Iím also going to take a wild guess and say that itíll be a bit difficult to machine since as far as I know nickel isnít known to improve machinability unlike lead in 12L14, etc.
I would recommend a good engineering plastic. Ultra-high molecular weight polyethylene (UHMW PE), nylon, or acetal come to mind. There are cheaper plastics like fiberglass reinforced resins and ABS but they aren't meant for machining.
If you get serious about this defiantly do your homework and compile a lot of data. I may be interested in helping from the CAD standpoint as Iím a mechanical design drafter by trade and have access to both 3D and 2D CAD systems.