check out YRM metal solutions online
they use 1.2mm Zintec for theit parts
i bought outer sill panels off them, thry were great, did a proper job
took me about a week of evenings to cut it all out and start again, but mine was worse than yours. i completely removed the outers, 90% of the inner sill was ok, but a few of the mounts needed reinforcing back onto the inner sill.
i used 2mm sheel for the NSR body mount, the mount itself was spot on but the whole inner sill and some of the floor had gone for about 6 inches into it from the back edge, so the 2mm sorted out the strength for the body to mount thing.
then the outers were fitted, and numerous small bots made up to box the ends off and reattach the pillars etc.
i also did the front inner wings, which had basically gone for about 4-5 inches in from the vertical section. the header tank etc was hanging on rust, the battery was 60% visible from the outside, etc etc. YRM do great panels for here too, but i took advantage of it all being small flat bits, so i cut and folded all my own panels up, again it was easy to make a nice job.
so it's all doable mate.
the rear crossmember that youre looking for is the body crossmember, not the chassis crossmember. this is visible sort of inside the rear bumper, and it is also directly below the tailgate. yes, mine's absolutely ****ed, but i patched it up to get it MOTd cos i cant be bothered to sort it yet, it's still acceptable structurally (obviously)
sorry for the essay, i hope it helps