The internet at large seems to think you can build your own Steam Machine. I hate to burst your budget PC bubble, but that's not actually true, and any attempts to do so will simply result in you assembling a rig that's substantially larger, not quite as sleek, and more like a traditional desktop than a console rival.
All the build-your-own Steam Machine conversations are part of an annoying tradition within the gaming PC scene. The arrival of a new console normally kicks off a ritual that prompts anyone who writes about hardware online to lure you in with supposed DIY alternatives. It's really no different from when the PS5 launched and a sea of voices washed in with advice on how to make a PC with equivalent specs, but the situation that Valve's cube has sparked is slightly different.