You are correct. "Vil ha" means both "wants" and "will have". It's a strange word, but Norwegian has plenty of these. I find myself asking my Norwegian boyfriend about these kinds of things all of the time, and he just says, "Yea, Norwegian is weird." Btw, I answered it the same way, and I got it correct.
"Har" is the present of the verb "å ha" (infinitive). You use "ha" if: it is next to an auxiliary verb (like "vil", "skal", "kan") or any other verb or if you are using it in imperative.
Jeg vil ha - I will have/I want to have Jeg kan ha - I can have Jeg har - I have Ha de (bra) - Have it (good) = (good)bye
(I'm not a native, but I'm pretty sure this is the difference)
No, one would never say 'an horse'. One would say 'an habitual occurrence', 'an historical text' and 'an heroic effort', however (but 'a habit', 'a history lesson' and 'a hero'). If you try saying 'a habitual' versus 'an habitual' etc., you will hear that the former sounds strained and awkward.
"an" can only be used when followed by a word that begins with a vowel in English. What makes it complex is that words like "hour" and "honor" take "an" instead of "a", because the "h" is silent at the beginning of those words (as well as some others). The "h" in horse is pronounced, however, so it is "a horse".
It actually is proper. In English the actual letter is not what is important for the word following the a or an, it is the SOUND of the word.
Historically, the sounds of the pronunciation of the word historic has been sufficient enough to warrant the AN. Either choice is correct. However, this is not the same for the horse.