Old laptops make good home servers. Built in displays, built in keyboards, built in battery backups, energy efficient. They take up a very small space, won't generate a lot of heat, won't generate a lot of noise.

All of this means they are much easier to live with compared to buying a used server.

Don't need a lot of CPU for most tasks. Max out the ram, slap in a large SSD, and you are good to go.

The weakest part of them is lack of I/O options.

However nowadays we are going to see USB-C based laptops starting to age out due to broken screen hinges, being out of warantee, and whatnot. So that is much less of a issue then it used to be.

So it is easy with those laptops to do things like slap on one or more 2.5Gbe ethernet ports, mirrored used "enterprise" 16 or 24TB drives for bulk storage on self-powered USB sata adapters, etc.

A alternative if you don't have a old laptop or you want to setup a small cluster for kubernetes or proxmox or something like that... is that you can get used business-grade "mini PCs" for prices ranging from $150 to $1000, depending on what you want.

A small "HP elite desktop" with i5 processor and 32GB of ram goes for about 300 dollars.

The trick to figuring out which ones to get is the generation of Intel processor they use. The newer the better. The i3, i5, i7 tells you the grade of CPU (probably want a i5 or better), but the important part is the generation of them... which is the first one or two digits of the the second number... like 'i5-8500' would be 8th generation i5 processor, which is "ok".

Right now Intel is up to the 14th generation cpu.

Also important is the generation of mini desktop. HP usually has the cheapest ones and they are like G3 through G9. G9 is current generation. And the, less important, is the "grade" of computer, like 400 is lower end, and 800 is nicer, etc.

So if you see a HP Elite Desktop G3 600 for sale and you can pick up a HP Elite Desktop G7 400 for the same price, then it is probably a much better idea to go for the G7 version.

Businesses often lease their hardware for desktops and when the lease is up they get sold for extremely cheap as "refurbished" items.

They stack easy on top of one another. Extremely small, quiet, don't use a lot of electricity, etc. Uses laptop-ish grade components. Come in a couple form factors.

Still have to buy a external UPS for them and have to figure out a solution for keyboards and mice.

For storage you can do something like get a full sized desktop and stuff it full of hard drives for DIY NAS/SAN solution. Setup NFS or iSCSI or something like that if you want something fancier.

And get a decent cluster setup that way.

The weakness compared to enterprise servers is that they have extremely weak power supplies. If you buy a mini desktop don't expect you can slap a big 'enterprise' HDD into it and expect it to work. Don't think you can easily add a big GPU.

Another weakness is lack of "lights out management". Ideally with enterprise servers you want a dedicated console network port on them (otherwise they piggy back off of main nic port, which is a security risk). That way you can remotely administrate them and do things like upload CD ISO images and whatnot to install and upgrade OSes remotely.

However enterprise servers are very hard to live with. They use lots of power and are noisy and hot. So unless you have a high tolerance for that sort of thing it can be a problem unless you have a dedicated room for a home lab.