couple of points..
2 - done and done! multi coloured/multi headed machines available now. Im sure i have seen one with 4 reels somewhere.
4 - what you are basically saying is along the lines of you want to write poetry but can't be arsed to learn grammar or spelling!
CAD is the easy part, autocad is no more or less complex than word, but FEA, which is what you really need, that can be a bit more complex. While the tech is fascinating, what you are trying to do is inherently complicated. if you can't stand the heat...
Don't know where you are getting your info on scanners from but +- 50 microns seems pretty accurate to me. and kinda the point of the current 3d printing enthusiast scene is actually making stuff you have designed, not replicating something that some other bloke/bloke-ess designed. so i would say the need for scanners is not so great. (unless making prosthetic limbs, where a model of the stump would be important!)
Especially as the materials and construction methodology do not make 3d printing much of a candidate for replacing _any_ component. An injection moulded nylon doh-dah is going to be far stronger than it's ABS laminar print lookalike. and if the nylon doh-dah broke in the first place, then it dont look good for the printed replacement.