You could buy a cheap celeron laptop with a large battery. They are preety cheap and robust. They usually run windows 10. They usally have only 2 or 4 gb ram that is sodered however.
I have a model with a 14nm dual threaded celeron that turbos up to 2.5ghz. The low power processor and massive battery allows it to have 10 hours of battery life when running my favorite game that runs on it. It has soldered 2gb lpddr3 ram with no expansion and 32gb emmc soldered to the board with no way to upgrade other than an sd card.
Because of this i would reccomend ram and storage limitation i would reccomend a used or refurb laptop with 8gb of ram and an older core i processor.
Do you already have the additional ram? You could find a cheap 4gb ram laptop and upgrade it. You may have trouble finding out if a specific used laptop supports an upgrade, and it may not be much cheaper. I would just suggest just getting an 8gb model. I dont think you can go any cheaper than around 75 without cutting down the amount of system ram.