Ask HN: Things to know when building a large CDN?

I'm working on a potential SaaS business idea and realized I'd likely have to operate my own CDN for static content on many of my customers' websites. It would be to serve my own content and cache and serve some of their content as well.

("Large" meaning "with the intention and possibility of eventually growing to be large".)

What are some tips beyond the basic things like trying to get the best price:performance I can for storage and bandwidth?