Spinoza spends the whole first part of his master book, Ethics, describing God and its properties, so clearly he thought there is something we can known about it.
He is often described as deist or pantheist ("God is the universe"), also atheist by some (he did not believe in a personal god), but he does not fit the definition of agnostic by any acception of the word.
As for the founder, it's difficult to say. As you said the term has been coined recently but the idea has been around since at least Epicurus ("we can't know the gods, so why care about them?") and I would be surprised if we can't find a presocratic with a similar view. I think the idea is fairly natural for any philosopher with empirist tendencies, and many can have reached the conclusion independently, which makes the idea of a well identified founder irrelevant.