Ray of light
 

I got a question about the ray of light item, if I understand the texting correctly once it's 100% reaches zero it will just disappear/be destroyed automatically, unlike other items who can be suppressed.

Just asking since that item will disappear from the store when when I turn in a quest and progress, and if it will disappear and leave my set incomplete I can probably skip hording up the money for it.

You`re right, item will stop working and wont be able to suppress it like other. So my suggestion is that save money get it, and use it only for very difficult undead opponents. As for set bonus ... there are other sets in game, get some that wont stop working in middle of game story. Gl, have fun :-)