You'll need SVN to download the source code to your PC from the live server, or (since it's not been updated in a while anyway), you can download the tarball: http://sourceforge.net/p/construct/code/HEAD/tarball
As for purchasing ProfUI, the reason is that Scirra used that software library to handle all the interface for the editor, and it's a proprietary and integral part of the source code that you would need to have a copy of in order to compile the project. I do not know which version is used in Construct Classic, but some other users here might know.
Edit: I also think this was coded in Visual Studio 2008, so you'd want to get a copy of that too (Express might work, I'm not sure as I have only compiled stuff from the Construct Classic Plugin SDK before).