With the upgrade promo, I can go to L5 Silver which includes the NAC for a little less than the NAC purchased separately.
I have the commentaries from my L4 base package (Bible Knowledge, Bible Exposition (Wiersbe), Opening Up, Teacher's). I have also purchased the EBC separately.
I am neither preacher nor Sunday school teacher. I use Logos for personal Bible study and as an aid in teaching the Word to my family. Not just devotions and the odd church service when the family is too ill to leave home, I also do printouts for my kids to complete as part of their corrections 
Anyway, do I have enough commentary in the EBC and the others I already own, or does NAC add enough additional info to make it worth it, given what I use Logos for?
A couple other questions:
This is my first upgrade, I purchased L4 3 yrs ago. Anyone know if I do go L5 Silver now, will I be able to minimal crossgrade to pick up the additional datasets from Gold? Or should I wait, do the minimal crossgrade first, and then upgrade to an L5 package?
I'm also considering potentially going for something a bit more expensive instead of the NAC -- the NIVAC. If I get just the NT for now, will it reduce the cost of the full set if/when I get it later? In general, do set have a personal price based on your purchase history or is that just limited to base package upgrades?