The best part is you don't have to run it inside ChromeOS, you can simply remove ChromeOS, enable the legacy boot loader, and use a regular old Ubuntu USB installer.
Of course, the models with more RAM are ideal, as well as being able to upgrade the M.2 SSD, but even a 2GB RAM/16GB SSD model will run pretty good, especially if most of your work is remote or on the cloud. Ubuntu 14.04 takes up about 7GB of storage.
I love older Thinkpads but these are cheaper, slimmer, and have more battery life.