How much rust is too much?

Gas mig, sheet metal, a roller, and assorted tools, especially air(Harbor Freight works). Then patience, time, and a learning curve. And ospho.
It's,not that bad, or hard. Just research the methods. I replaced a 1/2 rear 1/4 on my 70 Duster, learned. Presently me and a bud have replaced 2 1/2 1/4' s, and misc body on his 69 Charger- total of 32 hours per drunken ***. Looks good. The front and rear windows are next, this involves the roller/flanger tool. Parts made, but the real ***** starts there.
Once you play with scrap parts, and learn the tool; not that hard. Just cool the butt welds down with water, or watch the expansion of the metal.
Butt weld- leave a 1/16" or more of space, then spot slow, keep moving around, and wet rag. No gap( a weld is hot, then while cooling, contracts) No space? metal has to bulge, in or out. Practice off the car; get the hang of it.