Good guide, i agree for almost everything.
For Audio (in my opinion) you do not really need stuff from sennhaiser etc.... those are very expensive.
Personally i own a Superlux 681HD EVO (not the one in your photos) which costs about 30-40 (should cost 25-30 max but retailers have got gready because of how good these are, Superlux produces some parts for AudioTechnica
A cheap DAC with Xmos interface i've been using is Douk1796 goes for about 60-70eur and x2 JBL305 LSR as speakers.
Worth to check out the new brand made from JBL engineers (can't recall the name, tried everything really, they posted on reddit audiophile if i recall correctly and they are competing with JBL)
Stick to air cooling yeah but hold off from noctua. They've got too gready with their chromax and they don't really improve anything.
Dark Pro 4 bequiet, phanteks and criorig options perform similiarly and have better aesthetic depending on your liking. Thermalright should hell even perform better lol.
GPU yeah 1080ti or either AMD gpu, these hold longer compared to nvidia counterparts (check 7970ghz for example or 290x those cards just can't die).
I recommend Vega 56 with less budget than a 1080ti, they can be easily tuned to Vega 64 performance.
Chairs... i dont know, i am from europe and dont know where i could get these
Ah for CPUs stick to ryzen, cheaper, have more cores and upgradability. 2700x/3600/3700x + a good b450 board sounds like win for me, upgrade to ryzen 4950x when need more performance. lulz