I have original Windows discs sitting right here but for the purpose format/reinstall is there legal *up to date* Windows image available somewhere or is there some mystery method I am clueless about? (there have been 100s of MB updates since SP3 alone)
do you mean original as in SP0 so-to-speak?
If so, according to the nlite forums, you can slipstream it with SP3 (using nlite) and it should work. But I cannot guarantee that. I dont have links but did research it during the week. I actually tried it but it didnt work in a VM install - but I think that may have been to do with me adding the key in nlite and VMWare trying to also use a quick install system. Or maybe it just doesnt work...
There was an XP-up-to-SP3 download at one stage from MS -
I actually still have it I think, but it didnt work for my keys cause they are for OEM version.
Yes, I had seen WSUS Offline Update over the years at download sites but never considered using it, same with nlite. Have been hesitant to do OS reinstalls unless absolutely needed.
. Works like a charm. And you also have the old registry hives available for comparison(s). That process has been a life-saver (and a hair-saver) more than once 
.
Power user! Haven't heard anybody doing the side-by-side trick in a long time. Muy macho! (I like!)
) for me a few times too.