The "I don't know how to..." questions are mostly answered by doing exactly what you're already doing... playing through those main questlines at the top of the list. But don't hesitate to explore the secondary quests; there can be some good stuff there, too.

One of the things I like the most about NMS is that there's no real sense of urgency to any of it, other than what you imbue it with yourself... so picking and choosing whichever quest strikes your fancy at the moment can be quite satisfying.

Also, I suggest that you don't stress too much about units; if you can't afford something right now, just look into it later. Even if you never read a single spoiler, in time you will no doubt discover a few methods on your own to obtain an overabundance of units.

Mate, I've got about 200 hours into the game and I still don't really know what I'm doing. Keep following that quest as it really helps! I would say that there are lots of good money making methods out there. You can play the market and have a decent ship sooner than you think. If I remember correctly it might be more of a faff to fix up an a class. It might require a few things that are harder to find.

Have you built a base yet? They're also really useful and I think fleets are a good money maker too. Don't worry about buying a freighter as you will get a chance to get one much more easily.