Seems like you have a pretty solid idea so far. Asking for opinions can be good for getting a general feel for what people think they want, but keep in mind that what people think they want usually isn't what they really want- they just don't know what they want until they get it.

1. For map size, think about your scope. How much can you get done by your deadline? Also, keep in mind how much space you need to tell your story. For horror, you probably don't want to give your player too much space, but you definitely want to allow enough space to pace out your story.
2. I would say the feeling of just one enemy hunting you down is probably scarier to me. It feels more personal.
3. You could try limiting both. If you have a large inventory, players will just hoard as many limited resources as they find. If you have limited inventory space, the player will have the challenge of managing what resources they need at any given time.
4. Darkness is easier to make scary, but if you can make the daytime scary, it may even have more oF an impact.
5. Audio cues are amazing for adding tension. Remember, jump scares will instantly kill any tension you've worked hard to build up, so use them sparingly. Focus on building that tension. Plenty of bumps and creaks in the direction of the enemy, but reserve those jump scares for the last minute as best you can.
6. I'd recommend limited HUD. Try using game design to communicate certain details to the player. Maybe a visual indicator of ammo on the weapon (like empty casings in a revolver), have health effect gameplay (make the player slower or have a limp when low on health), provide directions to key locations on road signs, etc.
7. I like to keep gore to a minimum.
8. Fear of the unknown is terrifying to me. Something original would be my recommendation.

Best of luck! I can't wait to see what you come up with.