As far as I understand it, the only thing that matters for enchanting is the level of creature or human you soul trap. So it doesn't matter if you use a human soul or a creature soul.
However, nearly all soul gems only trap creature souls: only black soul gems will trap human souls. And black soul gems are pretty rare. Black soul gems (including the Black Star) work on humans of any level, including common NPCs like bandits. So if you're looking to expand your soul trapping opportunities, having Nelacar repair it is prudent.
However, siding with Nelacar will prevent Aranea from ever becoming a follower, which might not be what you want. While it's stated to only hold Black Souls (Humanoids) it can actually hold all types of souls, making it a straight up upgrade, as long as you're willing to go that way in the quest line.
In terms of a quest to collect all the Daedric artifacts, I'm not aware of any in game. However, if you're an achievement collector, all three platforms have:
Oblivion Walker
Collect 15 Daedric Artifacts
But both versions of the Azura's Star count for it, so choose whatever you want.
And as an aside, there is an Oghma Infinium quest, just not in the same style as Oblivion:
The reward for the quest Discerning the Transmundane is the Oghma Infinium, one of the 15 Daedric artifacts.
Vendors level their inventory as your level increases - and monsters get harder or become different. Harder monsters. Harder dragons. The loot you get from dungeons gets better.
Everything levels up with you.
Best Answer
UESP says 46 is the highest level required for leveled items. Level 48 is where you can start finding Daedric armor. We might need the Creation Kit to be sure about every single item.