I am yet to see actual images of this phone. Sounds like something I would buy, especially if I can install just a plain Debian release on it.
PureOS will no doubt be the best supported distribution but since the whole point of the phone is to be as open as possible and Purism aims to upstream as many of their changes as possible I expect it will be possible to run Debian with most functionality intact.
Myself, I really want to get NixOS running on it.
Let's start a working group, stat! What a cool story it would be if we could say that we built images for the thing before it ever even shipped and had a one liner to emit new NixOS-based images for a Librem 5.
There is so much potential for experimenting with update models, read-only secondary secure OS partitions (combined with Nix's reproducability starts getting pretty compelling, in my opinion).
I go by the same name and am persistently in the Nix(OS) related IRC channels. Cheers.