How clever are you? Do you actually need any help? Here is a bunch of stuff, feel free to tell me where to go if I am teaching granny to suck eggs.

Joining two bits of racking is easy, you just put a third piece on top an it gives you the perfect spacing while you nut up.

You want to cut 12mm steel, is that a specific job of a theoretical maximum?

An 8 x 4 sheet of 12mm is a fearsome thing. If you are actually going to be slicing 2mm while planning for 12mm, maybe you want a smaller, less vigorous torch to start with, or have you already been on CNC Zone and been convinced HyperTherm is the only option?

Possibly not because you don't have a monster water bed

It might be too late to mention that you do not need huge motors because there are no cutting forces. The only thing you have to push against is the weight of metal in the gantry when you reach a pointy corner. Weight is your enemy.

If you start using the same motor to lift the torch as you use to move the gantry expect derision.