"Do I need all these extras?"
Each module requires memory to run on your site
Before installing extra modules, you should consider the required functionality for your site, and only add a module if your site really needs it. Unnecessary modules consume memory and other resource needs of your site, and will make pages load slower. Because a module is popular or someone tells you that "every site needs it" doesn't mean that yours does. There are good sites that have been built with absolutely no contributed modules at all.
Also, you should focus on content, functionality, and configuration of your site before you consider how it should look (the theme), since that decision can be made later and may detract from your thinking about functionality and content.
Be selective
Themes are largely a matter of taste. For example, many people do not like to use a "fixed width" theme, but lots of people do. One nice thing about themes is that they are mostly independent of your content so you can switch them when you want. Caution, though, there are parts of your site (blocks, for example) that get built based on the theme, so switching themes can leave junk lying around to slow down your site.
Now, if you experiment with different themes and modules, you should also look at the Update Status page in the administration section and the Site Documentation module to make sure the modules are up-to-date and your site is clean.