Re: what kind of metals can i
Assuming you can get a welder to do it, the easiest way to make a good frame is to braze steel tubing. Steel tubing is easy to braze or weld, but welding thin-wall tubing tends to burn off some of the wall thickness opposite the welds, so brazing usually ends up being stronger.
Aluminum can be welded and brazed (with Durafix rod) but it is much more difficult than steel, because of the way that heat spreads out quickly in aluminum. Also some people have found that aluminum brazing rod does not hold well if it gets flexed at all.
Galvanized steel is never good for structural metal use for two reasons. First reason is that the zinc coating burning off while you are welding it is a health hazard, and second is that the chemical bath necessary to apply zinc coatings weakens the surface of the metal. So no good-quality steels are ever normally zinc-plated. Electrical conduit is steel, but it's a very low-grade soft steel.
In the US the main two metals to look for are 4130 cro-moly steel and 6061-T6 aluminum. Mild steel can also be used, but it is not quite as stiff and resilient as 4130 steel would be.
Anything else is either too weak (cast iron pipe) or a lot more expensive (stainless steel, titanium, ect). ....And aside from simply costing a small fortune, stainless steel and titanium are also very difficult to cut, drill and weld properly.