• t480 - dual RAM slots, dual batteries
• t480s/t490s/t14s - soldered ram
• t490/t14 - one bank soldered, one slot free

this is the list, nothing else, no Yogas, no Carbons. stay away from AMD models until the 5000-series, all earlier ones are a nuisance to run. feel free to get busted units - broken screen, bad keyboard, as working on those is easy, there are detailed hardware maintenance manuals available, for free and parts are cheap, plentiful and cross-model/generation compatible.

soldered RAM used be eww; nowadays with the prices, it's neat to get basically free RAM, provided it's at least 16 GB on-board. whichever you get will be plenty fine for the stated use cases. make sure you get a good battery, as getting those is expensive (boot off a live USB and upower -b)

remember, the goal isn't to get "a thinkpad". the goal is to get extraordinary value, in this case a discarded corpo workhorse that's still got plenty life in it, at a bargain price. if you're in an area where the stated models are plentiful and cheap, grab one. if they're overpriced, look for value elsewhere. e.g. Dell Latitude 74xx and HP Elitebook 840 are in the same ballpark.