Well, while I presume you meant to say less than 400$, if you really did mean more then I would highly recommend getting a Framework laptop. They are amazing fully repairable and upgradable ultrabooks, and they have mostly excellent Linux support (you can even buy them without windows preinstalled!). I’ve had one for several months now and it’s been great.
Yeah, Framework definitely needs to eventually make a less expensive device (right now they aren’t really price-competitive), but they are a small company with very limited resources, so it’s perfectly understandable that they haven’t yet. In any case, getting something like a used Thinkpad definitely still at least partially follows Framework’s vision while being less painfully expensive (I had some trouble justifying the purchase at first given the high price compared to the competition, although ultimately it was definitely nessisary to replace my aging Zenbook, and it will probably age better than the competition) and would probably work well for you (their reputation didn’t come from nowhere after all). Anyways, sorry for the long and somewhat rambling response (I need to work on cutting down all those parenthesis), and I wish you luck on your laptop-finding adventure!