This is about as cheap a setup that you can get that should run HL:A OK (Of course you could go a bit cheaper, but I don’t think it’s worth saving like $50). Obviously, the monitor and KB&M are just suggestions if you don’t already have them.
This build is meant for WMR or Rift S, if you want a build for the index, basically all you have to do is change the graphics card. I’d recommend the Radeon RX 5700 XT. Either this Gigabyte card or the Sapphire Pulse
If you have any questions, suggestions, something I forgot, etc, feel free to comment.