I second a ThinkPad with XFCE (such as Xubuntu) as the best combination.

A few things which, although obvious in retrospect, I had to learn the hard way:

• Get a ThinkPad with physical buttons under the touchpad, if you like to track with your index finger and click with the thumb. I made the mistake of getting the kind where you push the entire touchpad, and it's very hard to use. Apple's was at least useable, this is not.

• Get a ThinkPad with a physical Ethernet port. USB dongles are finicky and unstable on Linux, and there are places / desks / offices where the WiFi is flaky.

• Choose the stable release of a well-supported OS (Ubuntu, Mint... whatever) because the beta / newest / testing version will make you lose time on bugs and freezes.