$2000 - A loaded Dell Precision workstation. Wait for Kaby Lake quads though. If you like the Surface Books that's an option too.
$1500 - Dell XPS.
$1200 - ASUS UX501VW. (Only one RAM slot is upgradable, though)
If you have forgiven Lenovo after last year's man-in-the-middle scandal, they continue to make the best keyboards.
All of these machines run Linux well (with an occasional driver compile). The biggest tradeoff will be the touchpad. I find the trackpads on par with my 2009 unibody White Macbook. If you prefer a mouse like I do, it wouldn't be that much of an issue. On the plus side, you can get much better specs for the price of a Mac. Xeons, lots of ECC RAM, beter displays, better battery etc etc.