Best way to configure site

I’m new to WP, but PHP and MySQL are old friends. I want to set up a blog with multiple authors talking on different, but related topics. Each topic has sub-categories that are only related to the topic. For an extremely over simplified example:

Cars (the main web page)
Engines (author 1’s topic)
Fuel (category 1 for engines)
Gas (subcategory fuel)
Diesel (subcategory fuel)
Oil (category 2 for engines)
Synthetic (subcategory Oil)
Tires (author 2’s topic)
Rubber (category 1 for tires)
Air pressure (category 2 for tires)

I hope that gives you an idea. The engine author will want to blog about engines, fuel, gas or diesel. He will assign tags as appropriate for the blog so that it shows up in the different categories. So a review of a new diesel engine would be tagged with Engine, fuel, and diesel. The “tire” author will do the same and so on. The authors need to be able to create their own categories and tags, but the engine and tire authors are silos and will never cross tag.

The main http://www.cars.com landing page needs to display the most current posts by the authors, and a tag cloud of what the users are looking at across all of the author’s blogs. Each author’s blog would be on http://www.cars.com/engines or http://www.cars.com/tires. Followed by the subcategories http://www.cars.com/engines/category/fuel/diesel.

The user experience has to be seamless from one topic to the next. This is ONE website, NOT a collection of different sites sharing the same url. There has to be a consistent menu to get around and jump easily from engines and tires and their subcategories. Only one sign in should be required to post comments on any authors post. The theme will be the same from author to author except for the colors.

A multisite install seems like the best way to do this, but it also seems to be trying to pound a round peg into a square hole. Can what I want to do be accomplished by a single blog?