Hi Adam Preston,
What I suggest you to use is the trial version of the full RadEditor for MOSS 5.5.1. In order to download it you should go to the Free trials section under your account, click on the Visual Studio Extensions, DLLs, Scripts, Source Code, DNN and MOSS wrappers ... link and download the RadEditor for MOSS: (zip, 13 MB) installation. You can see the attached screenshot for more information.
You can find install instruction in the following help article Installing RadEditor in a MOSS 2007 farm
Also you can review the Using the RadEditor WebPart online help article.
If you decide to use the Lite version of the RadEditor for MOSS, you should know that it is based on the classic ASP.NET RadEditor and there is no need to extend your MOSS site with ASP.NET AJAX. You can use the help articles that I provided for this version as well.
the Telerik team