Question 1,2 and 3 all generally summed up into one: What gear to invest in?
It depends on your budget really, right now I would advise buying the new event gear, such as the Ancient Tree bark of brutality ( I think this is the correct name, correct me if I am wrong ). It isn't too expensive and is quite good for your level :), as you progress you really want to save up the gold for the Mythic armor set. The mage set is the cheapest and if you have and questions about it there are surely quite a few answers here in the forums. Upgrading an item with gems are definitely worth it because it increases a base stat of your character, you also have a chance to get a Grand gem which gives a larger bonus and tends to add value to the item if you are thinking about re selling it.
Rarity, as it follows we have the most common to the rarest, the rarest items are generally better and cost/are worth more.
Common<Rare<Epic<Legendary<Mythic<Arcane
The very best items are for the higher level players, Mythic armor is unlocked at level 26 where the HELM is available to be worn, the plate is viable at lvl 31. Mythic armor tends to be the best for damage and has generally good stats in multiple fields and is respectively the best.
Mythic and arcane weapons, Mythic weapons are almost second to none, Arcane weapons however are truly first place...
I won't go into too much detail but again, more information can be found on the forums.
Anywho, I hope this helps..no doubt someone with more knowledge will come along and give a comment with much more use but I hope this is enough for now..
:)