Any Lenovo Thinkpad X-series. For example, I am using an X240 to write this reply. They are very easy to work on, highly compatible, and very light and portable. You can find them refurbished on Newegg, or if you have a local computer recycling center, they may have a thrift store where you can buy refurbished laptops, and Thinkpads are popular among that crowd, so you might find a good deal there.

Alternatively, many of Dell's Insperon professional laptops are excellent, and in my experience they often have better quality displays than Thinkpads for the same price point (above HD resolutions, touch screens, and better color range), so definitely worth considering if that is important for your use case.