Suggestions for budget mountain bike with opportunities for upgrade
Assuming a new bike, look at the Scott Aspect 30, 20, or 10, depending on your budget.
They all use the same frame, and the only difference is components.
When you buy a bike with the intent to upgrade, realize you will spend far more long term, than if you bought your bike ready built with the same components you upgraded to. I understand why it can be better to do it that way to break up the cost, but it does cost a lot more money overall.
I would make sure you buy a bike with good wheels, a good frame, and a good fork. Those parts are astronomically more expensive to upgrade to than to buy on the bike.
Your budget is fairly low, so if you have the option, save a bit more before you buy. A budget of $800-1000 is more appropriate if you can afford to make it work. It allows the wheels and fork to be quality enough that upgrading the other components becomes worthwhile.
If you can make that budget work, then look at the Scott Scale line.
I like and recommend Scott bikes for you because they are readily available in Europe, and they make high quality bikes at a reasonable price.
I hope this is helpful to you.