You don't mention which language your scripts are in, but based on your other question about debuggers I'll assume you are using IronPython.
The most complete (free/open source) option that I've been able to find is SharpDevelop. It includes all of the features you've mentioned for IronPython as well as a debugger. It is open source and includes many useful extensibility points, so it should provide a good starting point to fix bugs and add additional language support if needed (provided your use conforms to its license terms).
If your situation requires use beyond what is provided by the LGPL then the best choice (though not standalone or free) is to extend Visual Studio (there's already an extension for IronPython though I'm unsure if it supports debugging).
It doesn't sound like you're looking to do much, if any, development of your own for this but if I'm misreading your question then there are some editor controls available that provide the features listed (and then some) for .NET (not exhaustive):
An example of a project that uses Scintilla.Net is Peter.