* making sure the motherboard supports the CPU via socket and also nominally
* making sure the heatsink can be mounted on top of the CPU socket
* having the case fit your motherboard form factor
* if there is a dedicated GPU, making sure it fits inside the case and there is a PCI-E slot with sufficient lanes for it
* if not, making sure there is an integrated GPU
* selecting a PSU that can handle the maximum power load for the GPU and that has a power cable for the GPU if it needs one
* making sure to buy the right type of ram in accordance with what the CPU and motherboard support
* if you want wifi, making sure there is built-in or there is a m.2 slot for wifi
* if you want a m.2 NVMe SSD, there should be a m.2 slot with the right key interface
Not often you end up buying parts that don't work well with each other and need to buy other parts. I once bought a small factor m.2 ssd only to discover it didn't fit the slot. I think it was trying to fit an M key device into a B key slot. Sometimes even the vendor data can be incorrect.
Quick read about what what CPU is compatible with what motherboard then selecting memory and sufficient power supply.
You have 6 main components.
-Case
-Power supply
-Memory
-CPU
-Motherboard
-Hard Drive
I'm really surprised someone on HN would feel like it is a challenge.