Selfhosted

I get that, that was also something I used to like about old servers, but let me float a few of the things that I've come to realize through my home-lab career to you:

• Raid is perfectly feasible in consumer hardware. If your motherboard doesn't have enough SATA ports, you can always get an HBA or a JBOD to support for more disks. There's really no good reason (that I have heard of) for hardware raid today. Just remember raid is not a backup :)
• There are consumer ATX PSUs with redundancy. However, the only reason for PSU redundancy is when you cannot tolerate downtime due to a PSU or UPS failure, and that redundancy might save you a few hours of uptime over 10+ years in comparison to a non-redundant consumer PSU that you can go out and buy if it fails. When was the last time you had a (reputable) PSU fail on you? What kind of uptime are you targeting? If you don't have an answer for that, 99% is very easy to reach even on consumer gear, and is a strong indicator that you don't need enterprise levels of redundancy. 99% is literally 3 days of downtime per year. Also keep in mind that redundant PSUs are just going to gobble more power and increase operating costs.
• KVM features - this was the big one for me. I wanted to be able to perform out-of-band remote maintenance on my servers. Then I took a leap and got a Sipeed NanoKVM, and I haven't looked back. there are plenty of them out there - PiKVM is another reputable one. When buying old enterprise servers, you often have to pay for the remote management license, and that is just another added cost. Not to mention that they lose support pretty quickly, and you end up running out of date software on one of your most critical interfaces to the machine. A NanoKVM, PiKVM, and others aren't built into the machine, so they continue to be supported for much longer.

One other thing that I'll mention and you probably already know - enterprise servers are LOUD - even just a single one can literally sound like a jet engine. That's not a hyperbolae. If this is your first one, don't underestimate it. I had my servers in the basement with decent insulation, I used IPMI to throttle the fans back to 10%, and I could still hear the whine on my first floor when everything is quiet. If you end up having to turn down the fans due to noise, you're going to start having heat issues, and then you're losing out on performance and shortening component lifespan. Noise-proofing a server is non-trivial - you have to allow air flow still, and where there's air flow, there's a path for noise too. My current setups all have 120mm and 140mm fans, and I can barely hear them when I'm working right next to them. My 3D printers are the loud ones in the basement now!

I don't think I've ever had a quality brand PSU go out on me. Software RAID like MD or ZFS works fine on basically any hardware, and I wouldn't use hardware RAID these days anyways.

I used to worry about that stuff and use enterprise hardware, but its just so expensive for decent performance, and so power hungry.

Like try and match even a budget i3-12100 or similar for single thread performance (needed for game servers mostly) and you really can't with used enterprise gear. Plus that i3 has an iGPU that can handle a ton of transcoding tasks, and ML for stuff like immich search or frigate object detection. And it uses about 10w or less most of the time.

I got a server for free and at one point, over half of my energy bill was for storing movies and nextcloud. Depends on the energy cost where you live, but for that price I could have paid for the highest tier of some cloud office suite that doesn't run slow like nextcloud, and the highest plan of netflix.

Of course i chose self hosting, but this time with energy efficient hardware, server now takes 50 watts peak, with 4 hdds and an nvme. Asrock rack motherboard, with IPMI.

Well then very little of what I said actually applies!

Unless you know the hours on a drive, you might get brand new ones, or you might get ones with 50k hours on them. They may also be from the same batch, which isn't ideal for data durability. If you're ok with all that, then go for it. I generally don't buy used drives because I don't want to take the additional risk.

I'd be surprised if you can't find a better deal on used spinning rust though... the shipping alone is probably half the value on a good chunk of sales from SmS.