as long as your chosen fish are all compatible, i myself prefer a larger tank. feeding 1 tank is easier, a larger tank truly requires less frequent maintenance, and the big one is i tend to pay more attention to 1 or 2 larger tanks vs a bunch of smaller ones.
lets cover some basics. the most important tool when building something is a piece of paper, and a pencil. draw everything out, with dimensions before cutting any wood. this will save on the use of a lot of 4 letter words later on. also measure twice, cut once. this will also save on the use of fancy expletives.
those should work. deck screws are made to take a load, and are therefore strong. unlike a drywall screw which tend to be brittle as they just need to hold sheetrock to a stud.
any straight 2x4 will do the job. use quality screws, not drywall screws. drywall screws are not strong. any good quality construction screw will do, i like the Spax brand myself.
the pictured stand, i would say no. in the pic it already appears to have a bow in it. it would probably fail under a 55 quickly, if it just didnt cause the 55 to leak.
soak it and do a few water changes. a day just didnt make much copper dissolve into the tank. you probably get more from copper piping in your house, and thats nothing.
ill jump in, sometimes i like to localize the ferts, but the fact is everything inside the tank is in the water column. if it is wet, its in the water column, so if you fertilize the water, everything in the water can feed from it.
keep all your substrate, rocks decorations in a bucket of water. the good bacteria will survive overnight. get new tank, set it up, fill it, treat the water. once it gets up to room temp it should be good to go.