I don't think you need to worry about a specific lowbie undead weapon, as you blow through the levels so quickly and low-level enemies die quick, regardless. I'd just look for the best <elemental> of ghostbane weapon you can find on the AH as you level up as a general go-to. And really, you don't even need the ghostbane part, except for the ghost-touch effect in quests with incorporeal types. Zombie and skeleton weapon-type DR can be a bit annoying on elite level, but beyond macthing the weapon you don't need any particlar types of extra damage to kill them quick.
Exceptions are for a few specific bosses where breaking DR can matter, and the ghostly skeletons in the Delera series :
- the Vampire at the end of church and the cult needs silver
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the Vampire at the end of the Shadow Crypt needs metalline (silver and adamantine DR), which is hard to find at level if you want the bravery bonus. He's hard to kill with straight melee anyway, spells are much better
- Liches need bludgeon (and sometimes good)
- Ghostly skeletons need good damage, but not bludgeon for some reason. And no worries there since you get the Club of the Holy Flame after finishing the first quest in the chain.