You'll like this part... you don't need anything.
The Site by Nite website builder runs online, so you don't need to purchase any software since it is provided with your website package at no additional charge. Because it runs online, you don't even have to install anything on your computer. Everything runs right from your web browser. Due to certain required functionalities within the browser, for now you do need to use either Internet Explorer or Firefox with the IE View extension.
This also means that you don't have to worry about software upgrades. In fact, you'll always have the most up-to-date builder and access to new features as they are developed. Given that most offline software for developing websites costs into the $100's and often needs to be updated at least every 2-3 years, this is a substantial savings and benefit.
In fact, while it would be a little more challenging, you don't even have to have a computer, as long as you have access to one. This however would be an extreme case that isn't recommended.
You can compose your content offline and then copy and paste it in the builer, or use the builder's built in text editor to add your content as you go.
You'll probably want at least a file on your computer to keep all of your original images, and perhaps a backup of all of your copy or text.
You'll of course need a connection to the Internet, and broadband is recommended (cable or DSL), but you can use dialup as well. If you are using dialup, just like any other dialup connection, there are increased chances of getting disconnected, so you might be better off writing all of your text offline, and then copy and paste when you do go online. This will also reduce the amount of time you need to be connected which may be less expensive due to connection costs.