Savings in battery. It’s mostly just from not running google play in the background. If you then go on to root that custom ROM you might be able to use some specific root tweaks. I suspect the savings for a custom ROM to be 10% or less. Airplane mode and battery saver will help a decent amount which is available on stock.
I might consider kodi since it will probably be more organized. Vlc will have normal file navigation but with kodi it will organize everything into movies and TV shows. If its going to be a purely offline device then you can just flash a custom ROM and see a decent amount of savings. It will likely be difficult to find a current generation phone with an sdcard and a tablet might make more sense for this use case. If you know you are gonna flash the device you are going to get you should make sure it has decent custom ROM support. Generally if it has an official lineageos build then it is well supported. All of this might honestly be more trouble than it is worth if you are not experienced with flashing a custom ROM so maybe just start with stock and see if you actually need to first.
It is unlikely that the service itself will delete your data. Although if it is one of those job board sites like indeed that try to build a profile and sell it to employers it might make sense to delete your account so your information is no longer forwarded