I third that motion. Doing similar with a T440p. I've spent $400 total between upgrading the CPU to a 4712 and putting 16g of RAM. Running WoW happily every night on Debian. Second hand Thinkpads are the way to go.
Not sure how to put it on a Mac, but I don't think any os is better than others. Windows, Linux, and MacOS to me is just like picking a flavor of ice cream. They all do the job, but what are you willing to tolerate?
Now I know that's over simplified but that's really how I look at it.
I third that motion. Doing similar with a T440p. I've spent $400 total between upgrading the CPU to a 4712 and putting 16g of RAM. Running WoW happily every night on Debian. Second hand Thinkpads are the way to go.