- How many enemies is appropriate per level?
Hard to determine until there is more of a gameplay element with them involved. Also depends on what kind of gameplay you want.
- Would you prefer a HUD?
Some way of signaling important state to the player is important. Those can be ectoplasm or health from what I understand you are going for. However it's worth considering if there are ways to display that to a user without a HUD.
- How strong should the enemy attack the player?
Similar answer to first question.
- How should the player attack? Would you like mouse click combat or full keyboard combat?
Considering the mouse is used to control the camera, it makes sense to add combat to mouse-clicks. However this also feels like a game that could play well on gamepad.
- How do you feel about the jump height/power? Should it be lowered?
Seems ok. But it depends on the usecase/game-feel.