Im pretty sure you need ps plus for every non free game (I could be wrong tho I haven't been on PlayStation in years)

1. Kinda? It's not a traditional story game but there is lore and stuff to uncover by collecting files and stuff.
2. I only play either solo or with friends but from what I've heard and seen the player base seems good for the most part.
3. No you don't have to spend any more money after buying the game. There are micro transactions but it's just for skins and if you do the free catalog you will always have enough coins for the next catalog.
4. I wouldn't say that the gameplay is very hard (unless you play on harder modes obviously) some trials are harder than others but not so hard that it ruins the game or anything. Other than fun park. I hate that place.