u can go for aluminum stockish to look for more option, some come with column and some with ribs......... much stronger........

2mm is more then enough...........

welding aluminum is difficult, not advicable............

well not necessary to use angle all the time, u can use profile, thick sheet at certain place to carry the load. just dun try to make things too long, and u can always make a small ribs to provide extra strength on heavy components supports........and speaking of aluminum, ask for 6000 series component for extra strength at PSU if u so worried about....

arcylic dun need that thick, a bit waste........count on ur structure is a better way to

Well you may not need to be a civil or mechanical expert, just use some common knowledge would be good enough............