I have three best-ever detailing purchases. These are supportive to the process rather than something that directly touches the car.
Pressure Washer -
Buying a decent pressure washer changed how I washed cars. I went from a weak Karcher K2 with limited connectivity to a Kranzle K1152 (1122). If you've never used a pressure washer at this level, you don't know what your are missing. A lot of that is the performance improvement, 6.0 lpm (1.5 gpm) to 10 lpm (2.64 gpm) is highly noticeable and makes rinsing more time efficient. But then there is the way it sounds, no high-pitched screeching and surging, just a stout and pleasant sounding tone. Then there is the superior hose, the ease of adding aftermarket upgrades, and the ability to service and repair as needed.
While I've since invested in a wall mounted setup, I still love using that 1152. Worth every single cent.
(Not trying to plug my channel, just showing the sound)
Sink -
This changed everything! Having a convenient central point to fill buckets, clean tools, fill foam cannons, and mix chemicals.................it made my workflow more efficient, more enjoyable. I always hated filling buckets with a garden hose, and mixing foam cannons meant a trip to the kitchen sink. The sink itself is nothing special, but its impact has been priceless.
Swisstrax Flooring -
Another addition that doesn't have a bearing on how clean I can get a car, but its impact was significant. I mostly did this for cosmetics, it easily covered up an ugly concrete slab with peeling paint. But then you start working on it and you go wow, standing in puddles of water after washing sucks. So, the standing surface is always clean and dry. I actually enjoyed the install process as much as the end result.
I know, those are significant purchases. But all three combined take a labour-intensive hobby and make it enjoyable. I've also sacrificed a lot to make those purchases; I don't drink, don't smoke, don't gamble, don't go to sporting events, don't eat out, don't take vacations, don't have children, don't buy expensive watches, jewelry or clothes. In the past, I've had people make snarky comments about my garage space, that it's not a place to do "real", and then take pot shots at what I've spent money on. Well, if they stop smoking, drinking beer and sitting on a sunny beach and maybe they could afford nice things too.