Guidance on Review Sites

Hi everyone,
I decided to have a go the product review market and have a few questions you guys might be able to help.
I am likely going with the WP platform. In the long run I want to provide product reviews on a wide range of products in all different types of categories from health products, electronic goods, pet products and so on. My questions are:
1. Should I create a new blog/site for every main product category or should I go with a WP Mu blog and have a sub-domain for my main categories with the main blog showing a summary of al the different categories like a directory? Or perhaps just a simple standard WP blog with a fully blown category/sub-category tree? Are there any advantages/disadvantages going one or the other way?
2. Any recommendations on a suitable plugin or WP theme? I want visitors be able to rate the reviews/products as well as writing their own reviews.
3. Is WP the best choice or should I go with a directory type software like for example Power Seek or something similar?
4. Is there much point trying to become an authority site for product reviews in general meaning having a site with a wide range of different product reviews or should I try to create many review sites in different main categories and pushing them to become individual authority sites?

Hope these questions make any sense and thank you in advance for your time and advice!