I purchased one just last week for the same purpose.
Newegg.com has Lenovo T420 Thinkpads on sale.
http://preview.tinyurl.com/ztwnyh5
I upgraded the Ram to 8 gig and installed a SSD. Purchased a USB 3.0 card for the express card slot (2 x 3.0) and a second cdrom caddy that allows for another SSD. It also has an M2 slot next the RAM on the underside.
Laptop was $199, MEM was $48, Caddy was $12, SSD was $89, and USB 3.0 card was $12.
The laptop worked fine the way it was but with the upgrades it is just awesome.
Was a little hesitant because it is a refurbished unit but when I got it, it looked brand new and the battery charged to a full 100 %. I'm really, really pleased with the whole purchase.
As for Distros, Mint, Ubuntu, Debian, Fedora, Kali have had no issues. Install was uneventful.