I have an S5 mini which I use for pretty much the same stuff (some browsing, music and messaging). I think you can get one for less than €200 online (I got mine

I have an S5 mini which I use for pretty much the same stuff (some browsing, music and messaging). I think you can get one for less than €200 online (I got mine for €250 about 2 years ago). Pros: Waterproof, good size, replaceable battery, SD card. Cons: Haven't found any supported custom ROMs.