Mixing ferrous and non-ferrous metals doesn't work. You can use normal mild steel for the curved bits and weld in Hardox where you think you need it. Normal steel will bend much easier (to put them into perspective, someone tried bending 4mm Hardox and the bender had to be set for 20mm steel for it to bend). 4mm steel should suffice for that body (May get a few dents but that happens )