- How many enemies is appropriate per level?
Depends on the play style I guess, if you're just suppose to clear all that are in a level you could play around with the idea that they might be hidden in certain areas (thinking Luigi's Mansion) and so in that case it might be a puzzle in of itself to find them. If it's a wave thing then maybe several.
- Would you prefer a HUD?
Similar to what others have said, it depends on the feel you're going for. The information should probably be displayed but whether that's a HUD thing or an environmental indicator like in Dead Space (maybe you glow red when damaged and glow a certain amount of blue based on your ectoplasm levels?).
- How strong should the enemy attack the player?
Again, I'm still not quite so sure what the end goal is so it depends similarly to the first question. (Can you heal)?
- How should the player attack? Would you like mouse click combat or full keyboard combat?
I guess clicking would be reasonable.
- How do you feel about the jump height/power? Should it be lowered?
For now it seems fine, but in what situations are you going to be using it (are you going to be platforming as well or can you dodge with it?)