I would say the X1 Carbon is not a real ThinkPad, and anything with Yoga in the name even less so. His brother's P series should be fine, though. Stick to X (but not X1), T, W, P series, and note that T is the "normal one". Also avoid s variants, e.g. T14s is not the same beast as a T14. Once you've filtered to this point you should rule out a lot of problems with build quality or soldered parts, though even then they don't really make 'em like they used to. Speaking of soldered parts, there was a bit of a dark age where even most of the normal ThinkPads had half or fully soldered RAM, but they're just starting to come back from that. T14 gen 5 is unsoldered, but gens 1 through 3 had soldered RAM, IIRC. Wikipedia has a table you can check for this. So, sadly the used market is gonna be full of the soldered models for a while.