Yep, making any major layout changes to a theme would require purchasing the Custom Design upgrade and then the willingness to learn the CSS needed to make the changes you want. You can ask for help and tips in this forum if you have specific questions.
Before you make your final theme decision, you may want to look around in the showcase to see if there's a single column theme that suits you. Nishita, Manifest, and Twenty Eleven come to mind.
If the Titan theme is the one you stick with, here is an example for how you might hide the sidebar and widen the content area using CSS:
#sidebar {
display: none;
}
#content {
width: 960px;
}
#content .entry.page {
width: 940px;
}
If you need a CSS tutorial, this is a good one to start with:
http://www.htmldog.com/guides/cssbeginner/