Maybe buys something that you can expand in the future by adding extra RAM, replace the storage and exchange the battery. A lot of the modern laptops don't allow you to do any of those (planned obsolescence?). I know older ThinkPads are a good option but I think newer models are less serviceable. In Linux you can use tlp to tune up your battery usage and reduce the power consumption to the bare minimum. I would also recommend a second-hand ThinkPad, but just check if it is upgradeable.