Had quite a few of the X and T series, X200, X201, X220, X230, T430 mainly, x230 would be my pick, you can quad-core mod it with the classic keyboard and use ivyra1n to flash the bios easily. I haven’t bothered with the Full-HD mod because the 720p IPS is fine to me, you can get them from Taobao or similar (Check sources!)
They’re all socketed CPUs, or you could get the chonky T530/W530 instead, or a P series. Old Thinkpads last a long time (although I have a bad habit of testing them :)
EDIT: MY T430 was also a fucking tank, it survived being thrown across a room in San Franciso with a tiny dent on the lid, no damage. They’re easier to Full-HD mod than the X series.