- Best practice is to use sprite characters
- For animation use a flipbook (made from a spritesheet with the animations)
- Their are youtube tutorials for locked items/powers, 3D tutorials can still be applied to 2D as its just 2D objects in a 3D space. It will be a little long to explain it in one message. But basically make a item/power, set a boolean that needs to be true to activate it, only turn on that boolean when you want the player to unlock it
- It wouldnt be a issue unless you had thousands of characters on the screen at once as unreal only renders in what you can see
The easiest way to start off is search on youtube for 2D tutorial in UE4: https://www.youtube.com/results?sp=mAEB&search_query=ue4+2d+platformer+tutorial