Ubercart

Hey Peter, I haven't done the ZenCart migration myself, but we have overseen some imports from homebrew systems and osCommerce... I don't have any solid pointers other than to say that customers in Ubercart are Drupal users, and address information isn't stored separately for customers. Ubercart pulls the addresses off of previous orders to fill an address book, though you can get a more robust address book using http://drupal.org/project/uc_addresses. I think the order tables should be pretty similar. There are others around who can probably be a little more helpful.

Regarding storing default shipping and purchasing preferences... there isn't a way to do this built-in. Ubercart will let you specify global defaults for options on product attributes... so what you can do is override these defaults in a custom module using something in Drupal called hook_form_alter(). You'd have to figure out a place to store these settings, though... one thought could be the core profile module. These are rough ideas, but my initial thoughts. Dunno how experienced you are w/ Drupal development.

As for the redirect... that's tough. It might be easiest if you're using the same node IDs for old product IDs and had a way to map URLs automatically... dunno what your prior URLs are like. And I'm no .htaccess / mod_rewrite genius. Could be you could just enter aliases into the url_alias table in Drupal. Something like that could be done pretty simply in the DB. Then you can use the Global Redirect module to make sure that only the right URLs get accessed. Or something... I hope that helps somewhere down the road.