You are over complicating it mate, focus on rifle, ammo, action, bullet selection and barrel if you are going to be making 1000m shots. Scope is important, but keep in mind that US army used 10 power scope for quite some time. Look for reputable brand at magnification you want, don’t over think it - your budget is not that big to get what you are visualising in your head… For example, x15 power - you will see a splash, you won’t see holes on paper at 500m for example. Don’t over think it, stick to budget - common, reputable brand and you will be fine.
re: shooting at lower then max magnification is often more comfortable. So there are use cases for that scenario.