Woot recently had the Latitude 7370 available, starting under $600 for a refurb. Near as I can tell, it's the same basic design as the XPS 13, but with Win10 Pro instead of Home and some more business-oriented features. (On closer examination, it also looks like it has a second Thunderbolt port instead of a second USB port.) I've not gotten around to installing Linux on it yet, but once secure boot was switched off, it ran SystemRescueCD and the latest Gentoo LiveDVD from a USB stick without any issues. Nice little machine. It's small enough that you can use it on a plane even if you're not in first class or an exit row...most other notebooks are so large that either the screen will be at an uncomfortable viewing angle or the front edge will be poking you in the chest.
Already took a look at the manual. Except for RAM (soldered on), everything's upgradable once you unscrew the bottom cover. Mine has a 128GB M.2 SSD; if anything's a candidate for an upgrade, that is, especially if I'm going to have it dual-boot.