Yes, you can do that either with categories or pages.
Originally Posted by geosite
Yep, you can. WordPress creates URLs automatically, based on your post title. You can, however, change the URL name to anything you want in your write tab.
2) Can I modify URL's? For example, if I don' like the URL MySite/World/Congo,_Republic_of, could I change it to MySite/World/Republic_of_the_Congo?
Yes, you can. The latest versions of WP allow you to keep revised posts as well, so that you can revert to an older version of an article and vice versa.
3) Would I be able to modify each post as often as I want? Rather than sit down and write an article about each nation, then put it in a database, etc., etc. I'd like to be able to simply visit my WordPress blog and add some information to my Mexico post if I happen to read something interesting about Mexico in the news.
You can use the same structure via WordPress as you're using now. You don't have to use WordPress specific styles. Styling is up to you and CMS independent. The site's design will look according to what you make it.
4) Would I have to alter the structure of my web pages, or could I leave them as is and just use WordPress to inject the content? If I have to accept WordPress' page designs, would I have a lot of latitude in modifying them - single-column vs two-column, various background colors, banners, etc.? In other words, I'd probably want to make the pages look similar to my existing pages.
You could use pages or categories, or a combination of both for it. I don't find WordPress to be overly versatile for these type of things, but you can do it easily anyway.
5) Could I use WordPress to create content for different sections, for example, with posts displayed at MySite/World/Arizona and MySite/Topics/Geography?
Which commercial version? There is no commercial version.
6) Would I be able to do all of this with the free version, or would I have to buy the commercial version? I'd like to try the commercial version, but it could get expensive, as I have several websites.