I bought a full 42U rack off a chap who was clearing out an office. Forty quid, collection only, "it's a bit heavy". That last part was doing a lot of work. It took two of us, a sack truck and several rest stops to get it from his car park into my garage, and by the end I had already started rationalising the decision the way you do when something is too big to return.
The rack itself is fine. The problem is everything around it. A garage is not a server room, and it spends a good deal of the year reminding you of that.
heat in summer, condensation in winter
The summer issue is obvious enough: kit makes heat, and a closed garage with no airflow turns into an oven. I propped the side door open with a brick, which helps until you remember the brick is the only thing between your hardware and next door's cat.
The winter problem is the one that actually worries me. Cold metal plus humid air equals condensation, and condensation plus electronics equals a bad week. I now run a cheap hygrometer next to the rack and a small dehumidifier on a timer. I would genuinely rather lose a few watts to the dehumidifier than explain to myself why a switch died.
noise is the tax nobody mentions
The 1U servers I started with sounded like a hairdryer arguing with a jet engine. Lovely density, genuinely unbearable acoustics. I swapped the worst offenders for Noctua fans where I could, downclocked a couple of the rest, and moved the truly screaming box to the far corner. The garage helps here, in fairness, because it is the one place in the house where nobody has to hear it.
what I would actually do differently
If I were starting again I would not buy the big rack. I would buy a 12U or 15U open frame, put it on castors, and accept that I am never going to fill 42 units with anything sensible. The empty space is not free. It taunts you. It says "go on, buy more servers", and that way lies a power bill that needs its own spreadsheet.
The rack stays, though. Partly because moving it out is as much hassle as moving it in was, and partly because, against all my own advice, I quite like having it. It looks the part, the cabling is tidy enough to photograph at a flattering angle, and on a cold morning the garage is the warmest room in the house. Regrets, then, but the kind you keep.