Complex classifieds system, how to start one?

Hi,

I am pretty new in Drupal and have a task to build classifieds site. Just getting into understanding into cck, views etc...
First I chose to try some of the frameworks like cake, codeigniter, zend and similar, but they are too much time consuming to develop even simple user anthentication so I went to Drupal.

My requirements are:

MAJOR:
1. Classifieds are in categories (auto, furniture, jobs, pets, technics etc.)
2. Categories can have other sub-categories inside (auto -> vw,ford,ferrari, jobs -> temporary,permanent,part time) and even deeper, without limiting depth.
3. Classifieds have the same data fields between them (price, main text content, city, contacts etc.) AND distinctive, depending on category: if user selects, that he wants to place ad in "auto" section, then he can select "fuel type: gadoline or diesel", "transmision type: auto or manual" if user selects to place ad on "jobs" then aditional fields like "job type: permanent, temporary..." and amount of monthly (houtly) salary...
4. Classifieds can have different states of their visibility, promotion status etc... For example, visible/template/expired/archive ads, and promoted/vip/vip of vip :) ads. One state, like visibility (template, normal ad, expired) is fully controlable by user, other state - also, but only after he payed through payment gateway...
5. Classifieds should have ability to change their state after some amount of time: for example, move from "normal" to "expired" state after 2 weeks, and automatically delete after 4 weeks.
6. Classifieds should trigger multiple actions after specified amount of time: for example send email to user 24hours before expiration and etc.
7. Photo upload for images - should be easy
8. Images with watermarks - found module
9. Various display types, ordering, filtering etc.. - views should help
10. Advanced serch: user should be able to search all the text AND only selected category/categories
11. Users should be able to modify their ads: edit, delete, promote etc.
12. Admins or other superior role user can do everything registered user can, also change roles of the users, change their permitions
13. Categories can have manually or automatcally edited "similar categories" lists.

OTHER REQUIREMENTS:
1. Limit user groups to some amount of ads (nodes) in period of time, for example 10 free ads in 24hours, other limits to "vip" users :)
2. SEO friendly urls - already know ;)
3. RSS generation - easy :)
4. Capcha, antispam - many modules...
5. Users adn anonymous visitors can write messages about inappropriate ads
6. Ads can be commented/rated - know how.

EXAMPLE:
I am trying to build a site, which is superior to this. It is in lithuanian, but you will understand main things.
http://www.skelbiu.lt/ title page
http://www.skelbiu.lt/skelbimai/nekilnojamasis-turtas/namai/ real estate->houses section
http://www.skelbiu.lt/skelbimai/paslaugos-darbas/darbo-paieska/ services -> job offers
http://www.skelbiu.lt/skelbimai/transportas/automobiliai/ford/ cars->ford, you can see, that data fields differ from the one in real estate
http://www.skelbiu.lt/3340998.html single car classified

So, my main concern is how to build the workflow, seems, that all presentation stuff will wait. With which modules I should start with? I know, it will not be easy out of the box solution, I will check http://drupal.org/project/ed_classified but I guess it lacks many of these features... I can join on developing some of "glue" modules for cck, views or to expand ed_classified, but I am not experienced to carry this alone.

Is anyone working on it? Any live examples????
Please conatct pm me, or this thread :)