To do on the cheap….
Cat1 – Bits to buy new as they are basically consumables….
Chainset (cranks will last for ever but since you always need new rings and BB – may as well buy all together)
Cassette
chain
tyres/tubes
cables
grips
Cat2 – Bits that will wear out eventually, and sometimes need servicing…. but you can get away with “nearly new”
Forks
wheels
suspension frame
headset
dropper post
brakes
derailleurs
pedals
Cat3 – stuff that doesn’t really wear out or need servicing and only dies when you brake it in a crash – second hand is fine
Stem
Bars
normal seat post
seat clamp
QR skewers
Saddle (possibly Cat2 if its a boutique build)
Hardtail frames (possibly Cat2 if its a boutique build)
Anything iv forgotten there……?