Most levelled items are no big deal: I pick 'em up, use 'em if I want to, and get something else when the first item loses its usefulness.

But I'm planning an Orc character who will specialize in warhammers, and Volendrung will play a central role. I don't want to rush out and get it at level 5 if it's going to be useless by level 15.

Should I wait until a high level to get it? Or alternatively, can I improve it (after taking the Arcane Smith perk) if I get it "too early"?

Wouldn't it make sense for an orc warrior to use common hammers and weapons for a while, gaining levels, becoming more powerful before being named Malacath's champion and getting Volendrung?

I say get a full suit of orcish armor, use an orcish warhammer or mace or something until level ~20

Daedric Artifacts aren't leveled.

This. Yes you can improve it (with the arcane perk) using an ebony ingot.

Here is a lot of the levelled items - http://www.uesp.net/wiki/Skyrim:Leveled_Items

As said above it isn't levelled
Also for an Orc warhammer using character you should conisder getting the Longhammer found in Liar's Retreat. An orcish warhammer that swings much faster than usual on DPS its a contender for best weapon in the game.

Huh. I just cleared Liar's Retreat today, no Longhammer. Is there a quest associated that I didn't know about/pick up?

No, it should be next to a corpse on an alter in the last part of the cave in the liar's retreat.

Well, the corpse and altar were there, but no Longhammer. Oh well, I don't use hammers anyway, it would have just gone into my "stuff I'll never use but have no reason to sell since I have 97k gold and nothing to spend it on" chest.