Regarding Linux compatibility Thinkpads are well supported. The firmware update process is supported by many models as stated above.
I currently have a “very used” x270 FHD, i5, 8GB 1TB SSD upgrade, WWAN/LTE and a supadupa additional battery pack. Bought it from ebay. Incl. HW upgrades (SSD, Battery) I spent less than 300€.
All works. I can recommend it as a cheap, 2nd hand buy. A highly portable, every day machine that just does it job.
The second laptop - a T14 MAD Gen. 1 - works out of the box despite the WWAN module Fibocom 850. Be aware to NOT buy the 850 model. If you search for xmm7360 and fibocom 850, there are lots of complaints that theres only alpha drivers and very poor support. So just do not spend money on it.
But though the rest is working perfectly I would not buy a T14 (at least Gen. 1 from 2020) once again. Performance with Ryzen7 is excellent, 400 nits FHD display is brilliant, keyboard a class of it’s own - but poor build quaity as sharp edges on front, very bad sound and webcam, just 6h battery life (I expected somewhat more when I bought it…) - it was too expensive that time.
If you’re looking for a used machine: thumbs up for x270 with FHD display
If it should be new I’d now buy a T14s if I need a top notch/prive/value Thinkpad.