Hey Jean! I see this is an older post and perhaps you've already found an option, but I've actually been looking into this--
Its upgrading (or getting an upgraded) Mac Pro 2009/2010.
I've been at various studios where they still use these towers and I've seen people run intense mix sessions without an issue.
This way you're still in the Mac platform, only downside is I believe Apple has stopped issuing OS updates as of this year for these macs.
But having said that, if you're fine with sticking to an older OS, you'll get a powerful computer for about $1200 max.