An 8th gen Core i3 should be absolute minimum if you plan on doing larger projects or rendering. I would say that an 8th gen Core i5 or a Ryzen R5 would be much better for CAD. If you only ever plan on doing very small projects, you can probably get away with one of the hyperthreaded Pentiums, but my recommendation is the i5 8400 or the Ryzen R5 1400 (lowest price for 4 core 8 thread performance) or if you can spring for it the 1600.