In terms of Ninja Tools, you want to look at their bonus stats. Purple rarity can have 4 purple stats, Orange can have 6, before that don't worry about it much you will level rather fast till you can start finding purple ninja tools.

As for upgrading, you c*e already leveled ninja tools for strengthening with no exp penalty (it will give as much exp as you put into it), so you can dump all your green and blue tools into purple ones later on. So feel free to max your green/blue ones.

They were sold for coupons in time-limited shop tab during event, so just wait for events that have them or they will be found in later team instances.

The only requirement is the level one to open slots, once it's open you c*e it, it shows you the level needed to open slot if you mouse over the lock & chains on it.

Ninja tools work on everyone, not just mains, so in general more stats = better. But imo defensive stats are better Life/Def/Res and for secondary stats Damage Reduction/Control/Combo.

I think the elemental resistance only comes in +1, i don't remember seeing higher so i would not bother. It's 1% off only one damage type and it comes in same slots as secondary stats spawn, i would rather have 20-25 Control or Damage Reduction there.