Freedom and security issues on x86 platforms
Opterons from 2011-2012 are still available and seem to be the best option to me for this purpose. They're reasonably performant (16 cores...), affordable and there are plenty of mainboard options. Software support is excellent of course. I'm just not sure how valid the "pre-2013 AMD is safe" claim is, since vendors have been known to include some remote management technology like Intel's ME in earlier versions before making it a standard feature.