Ubercart

1. It depends on how your static html site is set up. Drupal and Ubercart are incredibly flexible. You should be able to get something very close to your old site, but it might not be perfect. If you can post a link to your old site, we can provide a better answer

2. We are currently using oscommerce on several ecommerce sites. Zencart is based on (forked from) oscommerce, so they are very similar, so I feel very qualified to answer this question. Ubercart has several advantages over zencart.
First Ubercart is built on Drupal so Ubercart automatically has forums, SEO, and CMS features built in. Some of these features are available to a limited degree via modules for zencart, but they are no where near as good as the built in features of Drupal / Ubercart.
Second Ubercart has real modules. Zencart's "modules" are really patches which require editing the source code by hand. This gets really tricky (read "frustrating as hell") when more then one "module" affects the same file or God forbid, the same function / line of code. As someone with a heavily modified version of oscommerce, I cringe at the idea of splicing in the code for a new feature. Ubercart modules are simple to install. Just copy the module into the modules directory, log in ad admin, enable the module, and configure the options if necessary. I can not overstate how big a deal this has been in my experience.
Third, Ubercart has a core of paid developers. Ryan, Lyle, Shawn, and I are all paid to develop Ubercart full time (well, I don't work on it full time, but they do). this means that we address issues quickly, and are rapidly developing the cart. I also think this helps to insure a consistent, cohesive core of code.
Now for the bad. Ubercart has a smaller community of developers, fewer features implemented, and has not been around as long. All three problems are really one problem. Ubercart is a young project. But we are working hard to implement new features and the community is growing rapidly. There are some great coders out there writing modules for and improving Ubercart.
Second, because Ubercart is built on Drupal it has the potential to be a little heavier / slower then a pure shopping cart. I personally think this is worth it for the added features of working with a solid CMS. Plus, oscommerce is crap code. I think zencart made some improvements on it, but my gut feeling from running both oscommerce and Ubercart is that Ubercart is faster and places less load on the database server. Either way, hardware is CHEAP. You can easily buy enough ram to load your entire database for less money then it will cost you in time to try to maintain zencart in any sort of modified state.

3. Ubercart is very easy to customize. As someone who has been writing code since before the PS2, I feel that zencart is not in the same ballpark as Ubercart when it comes to modularization. Zencart isn't even playing the same game.

a) There is no auctionblox equivalent yet. We use auctionblox on one of our e-commerce sites, so I am familiar with what it does. Yes this is possible with Ubercart. I am certain that you could offer a bounty and get it done, or wait patiently and someone will get around to it.
b)You can certainly do this. Ubercart all ready has a pretty good import export feature you can use. As for the Windows program, it doesn't seem that difficult, but I don't think it is necessary. You can tell Dazzle to connect directly to your database tables. Dazzle can then get the info it needs for printing postage, and it will even write the tracking number back to the database so your customer can see it.

c) I think you would need to have one built. I don't believe the existing module will work with Ubercart. I will have to look into this a little further. Again, you can post a bounty and someone (maybe us) will write the feature for you.

Peace,
Andy

Andy, that was well written, you mentioned the pros and the cons.
The "one big problem" is causing me some frustration lately because
of some features like coupons and discounts not available in Ubercart 2.

I migrated our shop from OSCommerce to Zen Cart last year and now I'm ready to migrate again. Every shopping cart has it's issues but IMHO the must have feature for a shopping cart is a very simple checkout process with guest checkout. Zen Cart requires customer registration and has a 4+ page checkout. I know I've abandoned purchasing from sites that required me to register and create a password.
I was considering Magento which seems to have a lot of buzz at this time. However, I've noticed very little support in the Magento forums and I'm concerned with security issues. Magento has a Zen Cart/OSCommerce migration module but it was broken when I tried to use it.
Ubercart looks pretty good to me at this point. I've just started playing with the Ubercart on Drupal 6 beta release. I wish there were a Zen Cart/OSC migration instruction page. I'll slog through the node import and see how that goes.
--artbobo