My cutoff is if any of the structural wood is wiped out (i.e., rotted, termite infested). Most everything else is generally easily replaceable. It looks like T1-11 siding, which is easy stuff to knock out and replace if chewed up, and its not terribly expensive. Replacing the doors - heck, just use a few pieces of that T1-11 and make doors out of it. The shingles look to be close to the end of their life - You could replace those (and replace any sheathing underneath) without much effort), then go either with shingles, a metal/plastic roof, or even roll roofing (although that doesn't necessarily looks terribly attractive, it'll work). Either way, this looks like a great project shed. If it were me - if that giant evergreen wasn't hugged up next to it, I'd be dreaming of putting solar on it, getting some batteries, and putting power to it!