a. I don't have an immediate design in mind, but you can look for inspiration (or even download) from Open Designs. They have a lot of premium-looking themes available for free.
b. 10 sub-areas should do the job easily. 10 themes are too much hassle to integrate on the same site, and it's easy to cause inconsistency.
c. If your entire website is about displaying reviews of things (movies, books etc), then you should just do it all in one installation. Separate installations are for separated websites that need to use different customizations, or have entirely different topics / target audience.