Drupal and large sites

How is drupal able to cope with sites of around 40-50,000+ pages? My university are looking to move to a cms and are interested in drupal. How would it cope with such a large site? The multiple site feature from one codebase would be really useful to implement but so far I have never managed to make it work. For example if I wanted to make 3 faculty sites as follows: hums, natsci & med I assume I'd need to make 3 folders in the sites folder called hums, natsci & med with different database settings in their settings.php files. Once the drupal tables had been loaded into the database for each of them I am assuming the url for each would be http://www.mysite.com/hums http://www.mysite.com/natsci and http://www.mysite.com/med with the default drupal site being at http://www.mysite.com. Am I correct in these assumptions? Does it just work like this out of the box without any symbolic links/apache aliases/subdomains or anything else? Is there a step in there I am missing which is why I haven't managed to make it work as yet?

Is there a good site sucker module which would take squillions of html pages and mash them into drupal without much pain and effort? :-)