Chalice-Deathcap-Grail/Sorc-Void-Moar CDR - Games don't usually last this long.
Early deathcap increases wave clear to near perfect with e, not to mention the harass improvements.
Lux should, ideally, have max CDR, but aside from the grail second, you don't particularly need to rush it, you should have blue buff in the intervening time before morello's.
To be entirely honest : Lux's early game is NOT that important - Just keep up in farm, roam if necessary, and don't die. It's incredibly rare for a 1k+ damage aoe ult to be outscaled :)
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I like to go max CDR with the highest AP stack I can get.
This is, essentially, the best answer you're going to get on how to build Lux - While morello's does have better early mana regen, i'm a bit of a stick in the mud, and like my established build path.
However, as long as you follow the basic high CDR build with an early damage item, It's really a matter of preference as far as order goes, as she scales like a beast because of full AoE.