No matter what phone or phone OS you use, if you let it do the initial syncing and updating on mobile data, the mobile data usage will be through the roof.
The Nokia devices are fine, so is the Moto G series.
If you want to use your device for as long as possible, I would advise against getting the rock bottom. Afterall, you will still look at the display in 2021, don't get anything you are annoyed with today.
Whatever you do, don't go below 2 GB of ram and 16 GB of on-board storage. Hopefully combined with an SD card. It's not worth the 20€ you might save.
For saving data, just deactivate mobile data unless you need it.
Download the maps of your region while you are on Wi-Fi (use maps.me or here maps if Google solution isn't good enough).
At the price point, I have no idea what kind of iPhone you could get. All I know: do yourself a favor and don't get a device with 8 GB. Even with barely no apps, you will have to shuffle stuff around to make it work.