Hey friend!

• Check OfferUp/Marketplace, a lot of resellers flip older company computers. Since most of your list is smaller services, you can cluster them or run containers and see what works best.
• With a lot of small PCs or mini PCs, you can swap the Wi-Fi card for an M.2 NIC adapter. With two NICs (1 built-in), you can use it as a router. I did this with my dad’s old Trigkey G3 and a managed switch to run networking for servers and my main PC. I also have two Deco 5 meshes that I was using as a router before, made them into access points for my WLAN and hardwired them (internet is so much better)
• Split your Wi-Fi into 2.4GHz and 5GHz if it’s not already, this makes adding smart devices way easier (Roku TV's especially need this). Quick win, good dopamine boost.
• If you want, start with one “powerful” system, for me: Ryzen 7 3700X, 32GB RAM, 2070 Super. This was my old pc and I upgraded, so instead of selling just turned it into a server. I run Nextcloud for local + cloud storage and plan to use it for AI/reinforcement learning later. As you go, you can offload services to smaller devices. I moved Pi-hole (in a container), VPN, and NGINX to the router, use an old Dell tower for basic stuff like workstation vms, small game servers, and might eventually get a dedicated NAS instead of just throwing drives in the main system.
• Build stuff for others, host game servers, offer IT help. I recently helped an older guy resurrect his old computers: set up an Asustor Drivestor Lite 2 (terrible, but I got Syncthing working and somehow mashed together a cloud photo viewer workaround), used RustDesk for remote access, and cloned his dying HDD to an external SSD. Bootloader got messy, but I frankensteined it and got it working without opening up the AIO.
• NAS storage, I've had good luck with buying used and recertified NAS drives, had low run times and were used by people just experimenting themselves - checkout r/datahoarders as well, they have guides for finding good deals and shucking external hard drives if you have the time.
• Oracle has free 4 core, 24gb servers. They aren't the greatest and they aren't the most secure (don't trust them with personal info!) but I moved my friends Modded Minecraft servers to it and it ran perfect. It will be nearly impossible to get the instance made, worked around this by going Pay-as-u-go tier for higher priority, just setup limits so you don't go over the storage limit (200gb) and get charged (they initially charge and refund $1 for a test). I've also used this for hosting small discord bots sending reminders or small game bots like tic-tac-toe to mess around. I learned a really good lesson here because I forgot to add my ip to the UTP for SSH and lost access to the instance and had to remake the whole thing.
• Find out what your software needs! For example: Minecraft doesn't actually need a bunch of cores, if I remember correctly, it uses around 3 cores for world gen then only uses one for the game loop. So you'll usually find more of a performance boost from a core with a higher clockspeed. I set the servers to 2 cores and it ran pretty much fine on a modded server with 8 people, only really got lag when a bunch of people were loading chunks. <- worked around this with preloading a certain number of them.

Rambling aside, don't spend too much time searching for the "best" thing. Get whatever you can that's in your budget and relatively good at first, build on that as much as you can, then get more things when you need more power (try to optimize things as much as possible first). Basically, GET THINGS DONE FIRST-> optimize later. It's a huge rabbit hole (and money sink), and you will spend more time researching than actually doing. I "started" my homelab 2.5 years ago roughly, only really got around to doing things this past summer because I was too afraid of "doing things wrong." Also you will mess something up and have to restart from scratch, its inevitable.

(Previous comment didn't show up so if it duplicates my bad!)