Others have answered most of the questions, but my own two cents: Right now you should probably focus on Determination. While critical always winds up the best stat due to exponential increases, at the start of an expansion after rebalancing it starts out weak, only becoming godlike when ilvl increases allow you to stack more and more and more. Of the other stats that may be useful, WHM has great MP recovery options so Piety is not really desired, Spell Speed is of limited usefulness unless you keep your GCD constantly rolling (And if you do run into MP issues it immediately loses all usefulness), and Direct Hit, while more useful for DPS, does not help healing (By all means if you're confident in your ability to keep people up, focus on direct hit)

I know others have said not to bother with materia, but I'm in the opposite camp: Why not? You can remove up to grade 6 materia at a 100% success rate, it'll be a little bit of extra stats, and having materia bonded increases spiritbind rate, helping ensure you hit 100% and can convert it into more materia when you're done with it (Remember to take the materia out first! Also this is more useful later on when it'll turn into materia people actually use). If you have crafters leveled you can even meld it yourself for free, or you can request melds from a crafter friend.

As far as what food to use, it's honestly up to you. Any food will give a +3% xp buff, so you can use cheap level 1 food all the way for that. You *can* use food while leveling for stats, and finding the best food for your level is basically a matter of looking at the stat you want to focus on, applying the % increase, and finding something that gets you as close to its cap as possible. That said... consumables are rarely needed for leveling, and you'll likely find that they don't increase the speed of the run enough to justify the cost of using them. Just munch some raisins until endgame.

Also as far as potions go, it's the same as using food for stats but worse: As a general rule, stat buff potions are a pain to make as far as consumables go, and they only last a short duration so it's not like you can even keep 100% uptime like food. At endgame, using buff potions can help you push DPS checks harder and clear stuff you'd otherwise be unable to, but just finishing a dungeon a few seconds faster is an ill use for one... oh, and it should also be stated that basic potions, ethers, and elixirs are functionally worthless due to how little they restore and the massive cooldown attached.