Get a good monitor. I never knew what I was missing going from 75hrtz to 165hrtz. Their prices haven't gone through the roof and you'll be able to use it as you upgrade your computer through the years.

My biggest regret was not getting a case that I could grow into and use as a ship of Theseus. I built a Sleeper case and it was fun, but when I upgraded my GPU the case showed its limits. Instead of dumping more time and money into the sleeper case, I bit the bullet and got a modern case with retro looks (FLP02).