stu (nprofile…g44g)
The price/performance sweet spot for Thinkpads seems to Tx80 or Tx90 (where x is 4 or 5). I bought a used T590 for CDN$400 last week which so far seems pretty good. This was from a local brick-and-mortar shop with a warranty. EBay may get you a better price.
Because of RAM prices, your best bet is likely to get something with some soldered-in RAM and a (possibly empty) socket for expansion.