I want to use Orbs of Chance to get one of the unique onyx amulets, what base iLevel should the amulet be before I start chancing?
According to this post by Rhys, most uniques can actually spawn on any item of the correct base type, regardless of item level:
In general, the required ilevel to drop or chance a unique is the same as the base type. However, there are now exceptions to this rule, such as Auxium, Le Heup of All, and Atziri's Foible. These currently require an ilevel equal to their equip level.
As far as unique Onyx Amulets go, the minimum item level is still 20, as that's the minimum item level for Onyx Amulets in general.
Just as for normal items, the minimum item level of a unique item is the same as the minimum level to equip it. The unique Onyx Amulets all require level 20, just like a normal one, so any Onyx Amulet you find could potentially be chanced into any of the unique ones (bar any exceptions set by GGG to prevent new copies from being obtainable).
It's kind of rare for a unique item to have a different required level from its normal counterpart, but exceptions do exist (e.g. the new Voll's Devotion Agate Amulet is level 32 while a normal Agate's is 16). If there's a particular unique you're interested in, look it up on the Path of Exile wiki.