a 100 Eur cheap Stagg Strat copy, made out of the cheapest wood possible, is it worth upgrading with parts worth 400 eur (good tremolo, pups, nut, etc.), or Would I be better off just buying a MIM Strat?
Buy a MIM strat, you'll have a better basis to upgrade upon later.

R.I.P. Les Paul, 1915-2009
Quote by Shinozoku
You have a walnut stop sign banjo-tar signed by MAB

˙ןooɔ sı uosɐǝɹ ןɐǝɹ ou ɥʇıʍ ƃıs ɹnoʎ uı ʇxǝʇ uʍop ǝpısdn ƃuıʇʇnd
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You deserve an Awesome Award for Awesome People.

Stop Sign Guitar? HELL YES!
k thnx, just figuring out which of my guitars are worth upgrading which arent't
In most cases I'd say upgrading isn't worth it. However, if you want to do it to personalize your guitar and whatever, go for it.