Drupal, YUI, and custom themeing

I have a site built on PHP and around YUI (Yahoo User Interface). I currently only use YUI to layout my CSS structures (floated column layouts) and would like to implement Drupal 5.3 (PHPTemplate) with these modules: blog, forum, contact, comments, aggregator, menu, path, and search (among others).

Here's my site as it sits right now: http://networkedinsights.com

Some info on my site:

• There are 3 different layouts used: 1-col, 2-col, and 3 col
• Each layout has the same header and footer
• The homepage uses a slightly modified 2-col layout
• The main nav is a ul
• The secondary nav is also a ul and is nested within the primary nav
• Both primary and secondary nav bars are placed in the header (tabs that are floated right)
• The 2-col layout is the most common layout using a 75/25 distribution (the 25'er is my right-column)
• I will convert my current product help pages into Drupal's forum module
• I will also have a "News" section which will "stick" the top 3 News articles to the homepage
• I will also have a "Blog" section which will also "stick" the top 3 Blog entries to the homepage
• All other pages of my site will be static
• I'd like to keep YUI in place as this CSS/xHTML framework does a great job of supporting browser rendering bugs for floated column layouts

My questions are as follows:

1. Should I build my new theme on an existing theme like Zen or should I start from scratch?
2. How to I set up and present the "Navigation" block for authorized users (viewing 1 and 3 column layouts)?
3. I would also like to use my new theme for the admin pages?
4. I display several, reusable pieces of content in my right-column ... how should I pull these chunks of HTML into Drupal?
5. In my 3-column layout, I don't have a body or a sidebar, how would I build this page-3-col.tpl.php?
6. Speaking of page.tpl.php ... should I write one robust page.tpl.php to handle all 3 layouts, or should each layout get it's own page.tpl.php?

I probably forgot to mention something so please let me know if you have any other questions.
Thanks in advance for the help.

_rs