If I were looking to build a lightweight metal camera I would not choose stainless because it is not dimensionally stable...at least in the thicknesses that would allow it to be light weight. Stainless is a heavy metal. I know because I have worked with it for over thirty years.
Aluminum casting would work better but this would require the ability to machine the material. Welding any metal is just asking for problems with tolerances. I would probably not choose to use aluminum either.
I would opt for titanium. It is lighter and stronger than aluminum and stainless. It would require machining too. One of the small "all in one machines" would probably do what is needed to build a camera.
In fact this would also be what I would use to build film holders since the ability to hold tolerances is greatly enhanced over conventional wood working equipment. If the machine can machine metal, it sure can machine wood.
Welding aluminum is usually done with TIG or MIG reverse polarity DC with high frequency generator. The weld is flooded with argon for lighter material and flooded with Helium for heavier material.
Welding stainless (especially light guage--22-18 ga) is best done with TIG equipment ...DC straight polarity and high frequency generator. The weld is flooded with Argon to keep the weld from having inclusion of oxygen which will tend to cause the weld to become brittle and subject to failure.