My take on your questions:
1. There is no such thing as "plain old unix". There are variants on a theme.
And why buy when all the great ones are free. You buy when you want more support than you get from reading man pages and asking question in places like this.
2. I'd suggest you get solaris, open solaris, or CentOS and run it on you pc.
Solaris is more mature. CentOS is probably the best of the Linux types. After all, it's built on Redhat source
I run both types on a variety of systems. I'd would install both on your PC.
3. Keep the stable system stable. If you are doing dev work, then you can expect things to go strange sometimes. So keep the Mac running stable and try out the other systems on your PC.