I plumbed for the standard Stretch 2.7 64 bit linux distro. You don't need much to get linux to run, but it needs to run well enough - there's a kernel utility to monitor jitter on the real-time scheduler... On a little i5 (Viglen Genie) that's rock solid, other end of the spectrum with a thin client (AMD Dual-Core T56N) it was useless - at least out of the box. Bear in mind that your real-time performance becomes more critical with parallel interfaces and particularly using a software encoder set-up for spindle position (for threading). I wouldn't aim too low in terms of machine spec. Also, think if you're planning to use Parallel (probably best for a quick intro to LinuxCNC) - in which case make sure that you buy a desktop with parallel port. If you do ultimately go Mesa - I'd suggest a dedicated ethernet port for that - but that's easily added later down the path (though compels you to a machine that will take expansion cards).

LinuxCNC will frustrate you, at least at first. For me I'm still frustrated (why oh why doesn't Axis GUI allow a preset spindle speed on a lathe - and yes, the documentation says it does... but it lies!). Oh yeah - get used to substantial parts of the configuration being dependent on your choice of UI.

Damn, just noticed 2.8 is out. Maybe that's a job for later (I don't care for the older Debian installs). Interestingly there's a new Pi install - I'm not convinced on the merits of a Pi in this role, though. Anyway - 2.8 - don't believe there's much to backup to roll the version/Linux distro forwards. I'd be interested in anyone's experience of this (my current "reconfiguration" of my machines is currently in the heavy lifting phase... be nice to play with the Linux environment instead).