So I did it, I bought myself a Thinkpad X220 in perfect condition with 8Gb of RAM and a 240Gb SSD drive. It's been a few years since I've had two computers at home, and now I'm going to have geek nirvana, a Thinkpad made before Lenovo dropped the best laptop keyboard ever made.
There's also a great selection of spares, you could probably build one. I chose the 12" X220 because it's meant to be used on my lap in an armchair, you may find that the 14" T420 is a better choice, it's of the same generation and build quality.
There are lots of online resources for the X220, for example :