Site Architecture for Navigation Based User Experience

Hello All! Not sure if this is the right place for this post... if not- please re-direct me (and I do apologize).

I am looking for some advice/examples/tutorials/suggestions/etc..

Trying to build a site with drupal, and stuck on the core architecture. The desired effect overview:

*User can go to a search page and select country, state, and city.
*Within each city there are multiple categories, and 'posts' or 'nodes' that users can contribute. Users should be able to then search again by tags or types of posts within that city. This part of the structure will have categories, sub-categories, and posts in both or multiples.

The overall concept is similar to craigslist... but meant to be easier to navigate. **I would also like the application to remember returning users' city- again, similar to craigslist.

What I have done so far:

After several days of reading, researching, and module installations, I am stuck and crying out for help. So far I have installed views, cck, gmaps, faceted search, location, taxonomy, book, categories, panels, and others that have been recommended for people building similar apps. I am sure that the solution lies in all this somewhere? I keep thinking that I need to combine the book and categories somehow...

More about the desired affect:
*Users: set or change location and search for categorized posts within city. The can also post (I've got the posting bit worked out...)
*Admin: duplicate cities (with all desired categories), or states.. or countries... all in the same fashion. AND: if more categories are added under city- all cities are updated accordingly.
*User Navigation: There seems to be no module for dynamically updating dropdown forms- especially with a search function. I've seen some hacks- but I know no php. So, these don't seem like the best solution. I'd be happy with a sidebar or block that allows a guided navigation (and still remember the city per user- keep mentioning this to make sure that whatever I use to do it can tie to the architecture) but not sure how to set that up either...
*The Posts: the posts are location sensitive and category/sub-category sensitive.
*Scale: planning for huge numbers on posts and traffic (isn't everyone?) and growth/expansion of options

Anyone have something like this that can give me some advice? Or have the experience with these modules to show me the path to resolution?