I personally have the thinkpad e570 because there was a deal Convince me from buying a Lenovo E570 that dropped it to $600
The laptop I use is a dual core so if you want a quad core make sure to check for deals or see if any other forum members have any experiences with other thinkpads
you can also restore a old thinkpad like the video i mentioned
try looking at threads like this
T420 seems like a recommendation