Some is just the plural form of indefinite article "a".
E.g. I have a pencil vs I have some pencils. /. Alternatively I have pencils. Exactly the same meaning. No need to depend on any genitive forms in Russian.... the plural form of "a pencil" in English is quite normally "some pencils". Omitting "some" is an alternative option, with the same meaning!
Okay, I can get behind the concept of the plural of an item being "some" of that item without having to use the genitive, so I accept correction on that. I just still don't understand how saying купить карандашей (plural genitive) treats it as an animate noun - it is not a rule I remember seeing, and my original question in regards to genitive+inanimate objects has never been answered. From this site, you can use купить+genitive when talking about (uncountable) food or drink. But I would not see how купить+genitive doesn't turn uncountable comestibles into animate nouns, when купить+genitive turns countable pencils into animate nouns, and if there is a rule on it, I would like to know it. So that is really where I am hung up.
Because like that site you linked to says, partitive genitive is used for food and drinks specifically. In other words, карандаш can only be used in the accusative in this sentence, and therefore купить карандашей would be "buy pencils" in accusative case... except that with masculine nouns, only animate (living things or "life-like" things such as robots and dolls) would take on such an ending, and not pencils like you're suggesting.
That's a very ambiguous problem. I tried googling (as I personally don't see any difference) and other native speakers don't seem to know too. Some of them say "надо" hints a moral obligation as in "I have to call my mum [or she will be worried]", but I'd say "нужно" can also be used here. Others say "нужно" is a bit more polite. An explanatory dictionary by Ozhegov says they're equal by the way.
It is an adjective, saying that something is necessary, and like all Russian adjectives it has to agree with the noun:
Мне нужен стол
Мне нужна вода
Мне нужно море
Мне нужны люди
Нужно can also be used with verbs, like надо:
"Мне надо/нужно найти дорогу домой" : in this case дорога is feminine but нужно would be use because it is not the дорога that "is needed", but the "найти" ("finding" the road is necessary, not the road itself).