New editing computer

I need to look into a new computer for editing video. Mine is 2014 so it's 10 years old and constantly crashing. I have been looking at the new Mac Pro stuff that can edit 4k video and VFX work.  $25,000 is a huge amount of money so that's never going to happen. It's nice to dream once in a while.  

My friend Dale donated a 2009 Mac Pro 4 core that can edit 2k video but I haven't been able to find any editing soft ware that will work on High Sera 10.13.6. Maybe some one has an old copy of Premier pro that will work on it.  I tried installing the new one from their website but it won't install on the old system.  

My friend George donated a 20 year old server 24 core with the new free version of Devinchi resolve but it can't play back video with out chugging.  Any ideas?