I'm curious about this subject. I've come to my own conclusions that if I'm handed a 500k+ model that I'm not going to do shit with it.
If it's a simplistic shape, I'll simply recreate it from scratch and demonstrate the power of multi-res and subsurface division modifiers to show that you can have a insanely high vert model AND be able to toggle it down to a low vertex form where it's easy to manipulate.
If it's a high-vert sculpt, I just flatout refuse it unless I'm given a blend project file that is already utilizing the mult-res modifier.
I have a pretty nice computer, but like this guy says >>541094>Blender chokes when dealing with hi-poly objects
It's unbearable working with high vert models in Blender.