Controlling the number of blocks added to a region?

I have a region, with a template containing around 5 different blocks that the customer can choose from, to build their page content.

I plan to add another block that will let them add a small group of images to the page. But I only want them to be able to do this just once in the region. Anyone know of a way to only allow a <block> to be used once inside a <blocks> group?

Similarly, anyone know a way that I can check for the existence of one type of <block> before I allow another <block> to be used? In other words some sort of <block> dependency inside a <blocks> group?