I think you should definitely max cdr on Lux, but how you do it is totally up to you. If you're ap carry, you can blue buff + blue elixer to take care of the majority. If you're support, you probably won't get that blue buff. I take flat cdr blue's and quints, 9/0/21 masteries. That's just shy of 20%cdr, so building the other 20% isn't too tough.
Ap vs magic resist also depends on your role. If you are trying to carry, I think void staff + 15% mpen mastery is usually enough mpen. Then I build heavy ap. Deathcap + archangels + void staff is a ton of ap, and your nukes will hurt everyone. If the game goes long enough, a lichbane on top of this will give you outrageous damage.
If you're support, you can even ditch ap for auras if you really want, although I think most of us laser snipers still like the ap. Either way, I prefer to use lux's range to kite/harass. It's not uncommon that I can take a squishy carry out of the fight with a pair of lasers before the teams even engage, but that's just my playstyle.