How many enemies is appropriate per level?
I think that is going to be dependent upon how hard is is to defeat enemy i.e. you probably don't want a lot of enemies if each one is incredibly tough. This might be something that you'll have to play with until it feels balanced.
Would you prefer a HUD?
I think a HUD would be a good way to show how much ectoplasm you currently have. Additionally, since respawning was added I think a simple health system should be added and could be represented in the HUD. Everytime you get hit by a ghost or fall in lava, then you respawn at the last checkpoint. Once you lose all your lives, the game starts over. But this is just one example of implementation.
How strong should the enemy attack the player?
Again as in the first question. I don't think this is something that I'd be able to answer directly and is something that I feel would probably be left to more playtesting by the developers. However, I do think the enemies shouldn't be a one-hit kill or that it takes 1 minute to kill a single enemy.
How should the player attack? Would you like mouse click combat or full keyboard combat?
A mouse click would probably be sufficient. Maybe one or two extra keyboard key presses could be dedicated to a special ability.
How do you feel about the jump height/power? Should it be lowered?
The jump feels appropriate. It could probably be utilized as a puzzle/challenge mechanic. In the sense that the player has to jump between pillars posted over lava while also trying to defeat ghosts.