Is it hard? (He is a CS:GO and Mild WoW Player. I played WoW for 11 years, and Diablo 2 for 3 years)
It's hard for an ARPG. You need a lot of knowledge and experience to do well. Mechanics wise there is a big difference between a good and bad player, but there's not much of a difference between a good one and a fantastic one. There are some exceptions were skill makes a large difference, and that is racing from nothing and certain boss fights. A slow player might use 25 hours to hit maps in a new league, while a good player uses 12 hours and a great one uses less than 8.
Is it a complete story, or just a killin' things kind of game?
More of the second one. The story is there, but it's very out of the way. Think dark souls.
Does it have Competitive PvP? Or PvP at all?
It has pvp but it's not balanced for it. Build diversity is limited, few people play it, and your power level depends on how much farming you've done as much as your skill.
How is its PvE at End Game? (If any)
There are a couple of aspects to endgame.
The main one consists of a map system. Maps are drops that you use to spawn areas. You can use currency on maps to make them harder and increase drops. If you want to move up in map tiers (there are 15 total) you'll need to run harder and harder maps.
There are also unique maps that are special in some way (a huge vault with a ton of chests and no enemies, a timed dungeon that kills you if you don't make it in time with a special reward at the end, a single multi-phase boss, waves of undead, a gauntlet of submitted player characters, etc.)
Some people like to magic find. This can be done either in maps or by farming specific non-map bosses. You generally use certain builds that can stack item rarity and item quantity without too much loss of efficiency.
The third thing people do is farm Atziri. She is a boss with complicated mechanics and a unique drop table. You need 4 different sacrifice fragments to open her area, then she will drop a unique and potentially a higher tier sacrifice fragment. 4 different higher tier fragments will open up Uber Atziri's area. This is a much harder version of Atziri that drops some of the rarest and best items in the game.
What notable features does POE have apart from Diablo 2, or other games in general? (I DONT GIVE A SHIT ABOUT DIABLO 3 WHAT A PIECE OF SHIT HAHAA)
The most unique aspects are probably the passive tree and the gem system. I also really like the itemization and the endgame but that's subjective.
How many hours must i invest in this game to complete it, or become as strong as possible?
If you mean that literally, you could probably spend 5k+ hours maxing out a single character. The process would involve leveling to 100 (probably 600 hours for a new player), leveling gems to 20/20 and corrupting them for either 21/20 or 20/23, farming enough currency to mirror all the best in slot gear people have crafted, and getting specific corruptions of any unique gear you use. It's obscenely expensive, and very few people do it.
You can make a character that can do all the content in like 200-300 hours though.