This is a nice guide!
Used Thinkpads have been very, very good to me over the years. For most of the last decade I bought used Thinkpad T42's for $100-$200 (see the empirical depreciation curve here [1]), beat them up for a year or two, then swapped the hard drive into a fresh machine when I finally and inevitably killed the old one. No downtime, no waiting for repairs, no tears shed over the death of a fancy laptop.
If you're on a budget, or in situations where repair downtime is unacceptable (eg travelling and working internationally) it looks from this guide like a master + spare from the T4xx-T5xx series is your ticket.