^^^
This is all good info.

Above all else, play what you want to play. General rule of thumb is that Level 20 for battle classes is "capped subjob status" but the more you level up multiple classes, the more your character benefits and the more options you have to tailor your charater to your playstyle. Sentinel for the Ifrit fight is nice, but it is definitely not absolutely necessary.

Concerning flavor-of-the-month classes: we are set to get class revamps either at the end of the month or early next month when the next major patch hits, and the devs have stated that they are intending to make all classes viable in all major content.

Pick the company you want for personal reasons, you are required to spend time with all of them before committing so you can make a better decision. Don't worry about gear/location/anything. Story quests can be done with people from all companies, the only thing company-locked are caravans and company leves (the latter being solo-only so it doesn't make a difference).

Lastly, don't be afraid to buy things from NPCs when low-level gear is ludicrously overpriced. Also, you have the option of looking up the materials for gear and buying them, then shouting for a crafter. Crafters in this game, from my experience, are extremely generous and may not even charge you for the service. If you need to look up recipes, I've been using mooglebox.com.

Good luck! And if you end up not liking your server, you're welcome to join our small mid-core weekend LS on Istory