Built something fairly similar about 15 years ago. The cheapest way by far to do it is get everything you need off Aliexpress except the alu sheets which you can typically get as offcuts from local manufacturers, I was lucky enough to pick up some odd shapes that had been used for signage for next to nothing that I could cut to the right size. Since you need to buy bags of 50 or more of the M5 fixings, you an bundle them up and recover some or all of the cost by selling them locally once you've used the few you need.