> Can you select a file in the whatever is the Linux desktop equivalent of Finder and hit the spacebar to get a quick look at the file native to the OS?
Yes, I use pcmanfm on Linux and the spacebar will open the file in the default program.
> Can I run the software I need to be able to make a living?
Depends on what you do. If it's mostly design work and you require Adobe products then Linux is not a good choice. For software development then Linux is great.
> Can I run multiple HiDPI displays that I can connect/disconnect as needed without causing issues?
I never encountered problems connecting external monitors but also haven't tried connecting to an Apple monitor and makes me think drivers are probably non-existing for that.