wow, I thought my car was going to be hard graft to get right but that looks scary.
Interested in the rear subframe mount fabrication as I have yet to scrape or poke a hole in mine to see how bad it is,(hopefully not as bad as yours).
As for metal I bought 1mm mild steel for the floor pan but for the sills I would probably suggest something thicker like 1.2mm or order a set of pre bent ones as above.
The issue is even though you can't poke a screw driver through the metal you might not be able to get it clean enough(bare shinny metal) to weld to. I am having to cut back further so the metal is nearly pit free so my mig can actually strike an arc.
I bought stuff from this mob
https://www.themetalstore.co.uk/prod...ld-steel-sheet, it came well packed with no rust spots and wood/foam edging so it wouldn't bend/damage.
I stupidly bought 0.8mm steel too for doing wheel wells and stuff but its a bit thin and can be difficult to attach to the cars own metal without blowing through it.
Best of luck.