Unless this is an exercise in building a machine cause it's fun, buy another last gen HP workstation.
Otherwise I'd go with an am4 platform 16 thread cpu.
It's not going to fit unless you want to get ghetto with a Dremel or you have mad welding and paint skills to redesign your case. Even if you have the skills for the latter, I'd recommend getting a bigger case. It's probably going to be a lot...
It seems that's all the indie scene can produce. It doesn't require the same budget or effort to create those kinds of dated visuals. Despite these game developers having "passion" or whatever, I've never seen or played one that did anything that...