Depends on the exact country, as a local broker will (usually) handle taxes automatically for you. Of course you can do them yourself but in some European countries this could be time consuming and complex.

The EPA states that each American uses an average of 82 gallons or 310.4 litres a day (study from 2015). Source: https://www.epa.gov/watersense/statistics-and-facts

A good tip for weapons is to check out the market tab in game, go to the weapons section and sort by only show with blueprint available. It will show a good number of weapons where you can get the blueprint for credits and you can check if you have the necessary resources to build it in advance.

Otherwise, you will unlock blueprints for weapons during missions and quests. Also look on YouTube how the relic system works, that will explain how you get prime warframes and weapons.

Well yes and no, in the end they will have to clear every star chart node to progress so that is also an option as the reason, even if there is no specific progress towards a frame or weapon.

No it’s not necessarily bad, it’s a game after all and you should enjoy playing it. However, to progress and unlock all content you will need to level a decent amount of weapons and frames for the mastery. So try to get new frames/weapons as you’re progressing through the blinking nodes & the story quests. If you enjoy the new frame or weapon, continue using them, if not then just go back to one you enjoy, but try to always max the frames/weapons level before moving on.

Your main priority should be to progress through the star chart and clear all nodes, while doing that you will continuously unlock new sections of the main story, complete those as well.

This will slowly unlock all the (almost) endless content, game modes, frames and more that warframe has to offer.

Try out new weapons and frames along the way and always try to level something on the side so you keep increasing your mastery level.