If you are looking for a cheap phone to run Android custom ROMs (like LineageOS), you should look at older Motorola Moto G models or Xiaomi Redmi devices, rather than Nokia Lumias.
The Lumia 520 was built like a tank, and thanks to tools like WP Internals, the community has ensured that even a decade later, we can still open the hood and look inside. nokia lumia 520 custom rom work