HP does offer elitebooks without Windows pre-installed, at least they did last year when I picked up an 850 G5 with DOS only and no Windows. In previous years they also offered SUSE Linux pre-installed. I'm typing this on an elitebook running opensuse tumbleweed. Everything worked out of the box.
I've been using Kmail, KDE's default email client, for more than a decade. It integrates very well with online mail, address book, and calendar services. I haven't found anything else that does so many things so well. And of course it's perfectly integrated with the KDE desktop.