People may not like the idea, but you can get pretty decent laptops from AliExpress for like 250$, which was like 50% of the price of a similar laptop from any local sales channel at the time. I did buy one for my son when he entered university and it still runs well: 5-6 hours battery life, 8 MB RAM, some older generation core i7 inside, full HD screen. The touchpad and keyboard are not great, but after 3 years they still work. The specs today would be better, I guess.

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.

Check out Swappa.com for a used laptop… Got a very good deal on a thinkpad. Almost any laptop will work for what you do except for tinkering with almost everything which is kind of hard to define… Just avoid the Google Chromebooks

Preowned ThinkPad. You can get a 2018 or so X1 Carbon for $400 or less on eBay.

refurbished thinkpad or dell xps. (buying something like a business model could save you money on the long run, because you’ll be able to service the laptop later on. Instead of owning one of the new cheap consumer electronics that has everything soldered on and glued shut. And is generally made more cheaply.) Make sure to save enough money to afford a replaceḿent battery. The second hand one won’t be at 100% capacity any more.)