There are a lot of free and "bug free" themes; you can look at Wordpress's theme directory and even see working samples of these themes...
Most theme developers are smart enough to test their themes before they publish them.
Wordpress itself is free and I honestly don't think you should pay for themes unless you want something very professional and unique with more features than a normal theme...
You also have the option to create your own but you will need to learn HTML/CSS to style them effectively and if you know PHP, then you will be able to add all sorts of goodies to your theme