For years the homelab was a pile of mini PCs and a switch balanced on a shelf, held together by optimism and one overloaded extension lead. This summer I finally bought a proper rack, a second-hand 18U on castors, and moved everything into the garage. It looks wonderful. It is also a small catalogue of lessons.
First regret: the garage. It is cold now and it will be colder, and condensation is a real thing. Kit that idles warm is fine; kit that powers down and then sits at 4°C overnight is asking for trouble when it warms back up. I have added a cheap hygrometer and I am keeping an eye on it before I do anything clever with dehumidifiers.
Second regret: I bought rack ears and cage nuts as an afterthought, so half the gear is still sat on shelves inside the rack rather than mounted properly. It works, but it is not the tidy picture I had in my head. Cable management is the same story; I bought the rack, then discovered I needed brush panels, a PDU that actually fits, and far more velcro than seemed reasonable.
Would I do it again? Yes, without hesitation. Having everything in one place, on castors, with a single PDU I can actually reason about, has made the whole thing calmer to live with. Just buy all the boring accessories up front, and have a long think about where it lives before you wheel it in there.