I mainly brew 5 gallon batches. My HLT is an 8 gallon stainless pot. My mash tun is made from a 11 gallon water jug cooler with a copper pipe manifold (with slits cut into the bottoms of the pipes. I use an 11 gallon Bayou Classic stainless pot My brew kettle. I currently only have one BG10 burner that's in its own stand and I'm planning to use this burner on the bottom to heat the wort. I'm planning upon installing a BG14 burner that I was given in the top of the stand to heat the HLT.
As for using old bed frames, I know that they heat treated, but I've never had any issues with welding them. That said, none of the welds have been loaded that heavy. I was thinking of welding up a very basic stand with four posts, nothing too fancy.
The set-up I use now is basically a real "Rube Goldberg" and not the safest. Since only have a single burner I have to heat up the strike/mash water on the burner then lift it up about 4-5 ft so everything flows in the mash tum. Its a tad heavy and I'm always worried I'm going to scald myself. I've considered making a single tier stand, but I'm not a fan of all the pumping that requires.