Materias are for endgame entirely. Even then they were useless, but I think the last patch might have made them useful. But really, only IV and V would be useful, everything else is garbage, until they add a relic that needs them again. Crafting/Gathering ones are another story as those are valuable even the lower ones due to caps on gear, but combat stuff you can throw away. For the same reason, soulbinding low lvl gear is pretty pointless. You're better off trading them for GC seals at your GC once that opens up(don't remember when it is, if it's from the start or what).
On materia functionality, they're just like gems in say wow. You slot them, and that's it, they give you the stats. However they also follow item stats rules, so for example you can't have more main stat on an item so dex/int etc gems tend to be fairly shit. If the item has crit, you generally can't slot crit either, you'd need to slot speeed or accuracy or what not. Also only lowbie gear and crafted gear have sockets, so endgame you don't socket anything(but again I think they changed some of that in 3.1, not too sure as I haven't played).
Money is only repairs/housing/AH pretty much. AH is also mostly crafting/gathering gear or fancy looking gear for appearance. That said Relic quests tends to also have a bunch of gold sinks, so it'd be useful for that too once they re-add the relic in 3.15, maybe. Not sure what first step will be, not every step had gold related stuff. Just keep your money and don't spend it on useless shit, you really don't need to buy anything while leveling. You can buy a new HQ weapon if your current weap is kinda low level, but other than that I wouldn't bother spending any money.
Crafting is nice to have to repair your shit for a lot less(and anywhere), and otherwise is interesting to make money to buy crafting shit to make money etc, it's relatively self contained stuff so it's only worth doing if you like crafting, it doesn't really have any advantage on your PvE experience otherwise(other than the repair everywhere, which is pretty nice). It's also pretty expensive to do, so I wouldn't bother until you reach cap. As for how long, depends on how you do it and how you're leveling your main char. It's faster in general, but you can do crafting with like daily orders from your GC only and get solid levels over long periods of time like this with minimal time investment, or you can grind it out, or you can use leves as some sort of middle ground. With leves it's really fast though, as long as you have the funds.
Game gets kinda harder in higher levels, and raids especially, but the 4man experience tends to be fairly tame. Still harder than the leveling process though. Some dungeons have quests to do to get access but most shit is automatically unlocked(and those with quests tend to just be "go to the entrance and click the thing" type of quests. There's a healthy population at every level I'd say in the game, it uses cross server for matching and while queues can be shit as a DPS at times(like 15-20mins queues), it's still relatively fast.
For valuable stuff, can't really thing of anything special. Various monster materials can be slightly valuable, but you won't get large amounts while leveling so probably not that much. There's some dungeon equipement that isn't BoP though that you can sell on the AH for glamour purposes that sells fairly well.
