>>546guy who has bought a bunch here
They are nice and comfy, but when people tell you about upgradability and such they arent lying, but also realistically aside from the ram+storage you are unlikely to change much else.
Any i5 series should be good enough if you know what to expect and can lower expectations/optimise workflow well enough. the x250 should be good and better than the x240 since that one has no mouse buttons whereas the x250 does.
I would recommend the x260 as the best modern old one since skylake has great battery, but 5th gen should also be good, also in my experience the x260 has some design issue that causes freezes.
the t480 is a good bang-for-buck as it is 8th gen and thus 4 physical cores + usb-c. I have had issues with wifi on that one though.
the x series is typically 12.5 inch so you'll want a T series for 14 inch. Same again though, 5th gen and onwards is best. I would suggest looking at the screens in any online listings to be sure you get something you're happy with.
1366x768 is standard on a lot of them, if you're aiming for 14inch then imo 1600x900 is a good res to aim for, or 1080p if you cant get that