Are Eon Boxes a real-money "premium" thing?

I'm guessing the orange-labelled quests are Story Missions, does it matter if I don't do them right away?

Can I rest for a day to refresh the quest listings and farm lower-difficulty quests for credits and loot without missing out on anything important?

There are no real money features in the game.

Story missions will not disappear until you do them, so you can safely rest (you can outlevel them, though). Although resting is kind of costly.

Yes you can farm the side missions.

Instead of paying a rest to refresh those missions, you may easily cheat: quicksave and quit then reload.

That's great news.

I find this game a lot of fun, but I really suck at it.

Even on "Story" difficulty I get wiped out by things like bleed effects because I can't out-heal them. Oh well.

At least I can spend some time grinding money to hire new Crawlers while they're still relatively cheap.

There is few heal in this game, so it's better to protect the most, prevent damage.

Nobody in the thread mentioned it, so you should know that orange means somewhat difficult at your level. Red even more difficult. Greyish missions are quite easy. The rewards scale to difficulty. If you farm non-story missions, as your levels increase, the story mission that you put off becomes easier and downgrades in color.