I put libreboot on an old 2006-era X60s, and it wasn't too much trouble once I'd worked out which instructions to follow and which ROM to use (which was not obvious). That appears to be the best supported laptop, and you can pick them up very cheaply on EBay.

Picture: https://rwmj.wordpress.com/2016/04/10/libreboot-on-my-x60s/