Hello Rick,
You don't necessarily need different domain names to do that. I've got the same setup at www.edusite.us where I have six Moodle sites set up. You don't need domain names or subdomains if you don't need to submit the individual moodle sites to search engines. If you do, however, want to submit them to search engines you will need to at least turn each site into a subdomain. Everything I have read on subdomains and search engines indicates that the two get along fine.
Notice the top nav bar throughout all the sites. It is doing the very same function that you intend to use postnuke for but does in a simpler fashion if you don't need all the bells and whistles of a full fledge portal.
The menu can be customized for each site if you wish by using some coditional statements to decide which nav bar to post. The logic would be like this:
IF(this site = 1){ menu = "blah blah code for site one menu"}
elseif(this site=2 ){menu = "blah blah code for site two menu"}
etc, etc.
I'm not sure if the postnuke script will work for multiple moodle sites either. I think the code for consolidating the two was mainly created to eliminate the need for logging in twice, once to PN and once to Moodle. I think it would allow all the users to the five sites to have access to all five sites. You may or may not want that. My method has each Moodle site using its own original database so that users in one site only have access to that one site.
Regarding the quizzes, I've found GIFT to be really easy to use. It only takes a few minutes to create a course, assign the teachers and import the questions and answers using GIFT.
To share files between the sites there are a number of different approaches. Simple HTML links can be used in some cases. I also use the php include function to help cut down on the space and administrative load. Do this by making a small php file that includes the large one. You import the smll file into each site, but the large file remains intact, by itself and is the sole dource to the other files. You can then modify the one file and it will be implemented on all the sites.
I hope this helps. Let me know if you have further questions.