I love drupal. Really. But the templates are just full of cruft because they try to provide every possible class for every possible situation. They draw CSS from many places (sometimes if you let it have its way, you can end up having 6-8 CSS files included from all over the place).
That said, if you aren't fussy about having lean, clean HTML, it isn't bad to modify a drupal theme.
A good way to start is to use the Zen theme, follow the install instructions and you'll end up with, within your drupal directory (i.e. web root in most cases):
Within the "your-theme" dir (which is obviously named something more appropriate), you'll have a bunch of files, but most notably
layout.css (essentially sets the wireframe layout)
your-theme.css (typography, overrides for other styles)
page.tpl.php (the framework HTML)
Modifying those files to import the HTML and CSS from your current pages will get you pretty close to where you want to be. Because your-theme.css is loaded last, anything you put in there should override defaults elsewhere, though on rare occasions you'll need to put in a !important.
Hope that helps.