How do I integrate queries to legacy databases?
Welcome to Drupal (and also Drupal answers) :) Glad to hear that you've chosen Drupal.  

Although the scope of this question is pretty big, I'll try my best to answer. Feel free to ask more questions.

You might want to first check out whether Drupal fits in your requirements. You can check Case Studies of different sites which were built (from scratch or migrated) on Drupal. See if it gives you any idea.  

If Drupal does fit into your requirement (mostly it does), you can check if there is any Drupal distribution present which is closer to your requirement (e.g. Commerce Kickstart distribution is a customized Drupal installation which is modified to provide features for a typical e-commerce site; while OpenPublish distribution is for news/articles publishing).

You don't need to develop a module per page. If you come across any requirement, there is a popular saying which goes "There's a module for that", which just means that there is a possibility that someone else who faced the same problem came up with the solution in the form of a module. You can search the list of modules on the official Drupal.org site, or drupalmodules.com, or just google it ;-)

You might also check for the Migrate module, which provides out of the box support to migrate sites powered by other technologies to Drupal (with some customization).  

If you get any queries in your Journey to Drupal Land, you're always welcomed here on StackExchange's Drupal site :)