Lenovo's thinkpad line are the only laptops with decent keyboards, and the ones on the older thinkpads were better.
If you're not compiling anything big and your biggest worry is keyboard quality, then getting a used t420 would be a good idea. Thinkpads have the best build quality you can get, and most of the used ones you'll find online come from businesses who are upgrading their equipment, and therefore are ussually taken care of very well. I've bought many used thinkpads before, and they're all great. Hell, I have an old one from the late 90s still running windows 95 that runs like a charm even after years of abuse. But, yeah, the t420 was the last thinkpad to have what i'd consider to be a true thinkpad keyboard, i.e. not chiclet and the right amount of rows. It has the best hardware inside that you could get for a laptop when it came out, which wasthe time sandy bridge was released.