Hierarchical Structure or Not?

I have a very specific question regarding navigation optimization. We have a site in which our products are organized by Vendor (~6), Model Line (~60), Model Series (~900) and Models (~20000). We have a few hundred thousand parts that have specific part numbers that are the most critical to get indexed. We are in the midst of a redesign and struggling on a navigational approach:

Option 1:
Home Page (List of Vendors) > Model Line > Model Series > Models > Model Part List > Part page

Option 2:
Home Page (List of Vendors) > Model Lines > Model Series > Model Series Part List (all parts from all models in the model series) > Part page

The default list of parts for option 2 on the model series part list page is all parts for all models in the model series. To help the user determine the parts that are applicable to his/her specific model, we were thinking of implementing a filter option (i.e. drop-down list that you can select your model).

Pros/Cons of Option 1:
Most useable format for customers because it leads them through a very logical progression. This format also minimizes the number of links on each page level which is a plus for internal linking because the fewer links you have on the page, the more page rank of that page gets distributed to the lower level pages. The problem with this structure is the number of levels it creates. It is our understanding that with each logical level in the site, Google Page rank decreases by 1. The most import page, Part page, with the most important keywords, part number, is located 5 levels down. Which means if you have a page rank of 5, your part page will at best have a page rank of 0.

Pros/Cons of Option 2:
The intention of this structure is to minimize the number of logical levels so we can increase the page rank of the part page. With this structure the part page is only 4 levels down. However with this structure, many of our model series part list pages will have 600 plus links to parts.

Questions:
1. It is our understanding that the more links you have on a page, the less page rank is passed down from the parent page. Therefore is it better to have more logical levels or fewer logical levels, but more links on the page?

2. Is there any limitation by the search engines on the number of links on a page? I know Google has a page size limitation (~100K).

3. We have a solid understanding of how our users will use the site and therefore we always build with that in mind. However, with customer useability aside, what would be the best seo approach to take with a site of our size and magnitude?

4. Should we create a site map that links to all our part pages directly, i.e.
[Site Map]
[Part Site Map 1] [Part Site Map 2] [Part Site Map 3]

Where each part site map is within the 100K Google limit.


Any suggestions/comments would be greatly appreciated!!

Thanks,
Calvin

I suggest Option #3.... Design the structure for Battle.

If this were my site I would let keywords determine the upper level and intermediate level divisions. I would do my keyword research and let that guide me - this research will determine the title tags of the site - the parts of the site that engage with the search engines. Structure your site in a way that allows you to write WINNING TITLE TAGS and undiverted SEO. Your site must succeed in the SERPs to attract visitors - and once visitors are there they will tolerate a couple of clicks to get where they need to go - if the decisions that they must make are easy.

So don't create categories that split or combine your keywords. That will force you into a compromise on your title tags - the most important words in your entire site. Write them first and then build the site structure around them.

After you have this determined the structure then design your pages, giving prominent real estate to H1 and navigational linkage, etc. Most important is to include some ssi blocks where you can direct/redirect your internal linkage much like a general would direct his troops in battle.
(this will be a bit easier if your site will be dynamic - but I am currently finishing a site of about this size and it will be html files on server so I have total control over filenames, etc).

On my sites I also include a few ssi blocks that can be used too to change optimizations within categories. Design around Title tags and SEO because you MUST win in the search engines first before anyone will have the pleasure of seeing your products.

EGOL,

In your opinion, how would you do this without sacrificing usability? Where is the line?

-Greg

A low conversion rate on a well ranked site will sell a lot more units than a site that sits at #347 in the SERPs with perfect usability.

If you read the book "Don't Make Me Think" by Krug, you will learn that site visitors generally don't have a problem with taking a few clicks to get to their destination - if you give them an obvious path.

Spend the time, hire a usability expert, read some books... if you have traffic you can improve your conversion rate.

Which approach degrades page rank more, number of links on a page, or number of levels in the navigation? Specifically, if I have over 100 links but in order to have fewer links I have to build an additional layer in the navigational structure, and I want maximum page ranking at my lower levels, which approach should I take?

Calvin

I agree with EGOL - I'd rather be #1 in the results and be a more difficult site to use than be #394 and be simple.

It shouldnt be hard to do both though- Put your keyword catagories in plain text as links instead and make the menu easy to use, that should work fine too.