I hope you have money. A lot.
Also, depends on the part and the tolerances needed. Of these choices. I would tend to say the micrometer/caliper is the best as -no offence inteded, but since you're talking about making a 3D scanner, I don't think you' (or anyone without some serious tool) wll be able to achieve sufficient accuracy for CAD use...
Last edited by Conker; April 18th, 2011 at 23:40..