What is the minimum eye?
I think that you are expecting for some complicated system while such things are not necessarily involved. You basically just need a protein that is light-sensitive and some cascade of signal transduction. A light-sensitive protein is just a protein which conformation is changed in the presence of photons of a given wavelength. This cascade of signal transduction can be very standard. It doesn't need to be anything extraordinary.

Consider a mutation that turns a given protein into a light-sensitive protein. Such mutant protein could easily still activate the same cascade (and eventually regulate the expression of the same gene) and may eventually be beneficial. A visual signal (presence/absence of photons of a given wavelength) directly affect the conformation of the protein which in result can activate a whole cascade. The cascade doesn't need to have evolved after the protein had acquired the ability to response to photons. You may want to dive into the details of how the structure and function of the opsin proteins.

This article describes how an ancestral non-light sensitive protein (coded in GPCR gene. The proteins are also called serpentine) evolved into a light-sensitive protein (opsin). The article also link to other articles that discuss this transition.

There are several types of light-senstive proteins in different kingdom, including plants and bacteria that don't have nerves.

You can learn more about the cascade of transduction on wikipedia (here or here for example). You may want to learn more about opsin proteins and learn more about light-sensitive proteins.

Wikipedia page: Evolution of eyes > Early Eyes will probably interest you as well. Depending on the definition you use for "eye" (note, "eyes" have evolved independently 50 to 100 times), Planarian might be a good example of a species that have a primitive eye.