First a few clarifications:
- A .csproj is an MSBuild project file, as are most Visual Studio project files (including WiX projects). A .sln file is not an MSBuild project file but MSBuild will convert it to one.
- As of its most recent version, MSBuild is no longer part of the .NET Framework. It is now in MS Build Tools, a separate, free product.
- Even with Visual Studio Express, you have full use of MSBuild. Express does not allow non-Microsoft extensions to Visual Studio. The WiX Toolset does provide both MSBuild targets/tasks files and a Visual Studio extension. So, with Express, you don't have the WiX project template expander and editor.
An alternative to Visual Studio is SharpDevelop. It provides its own WiX project templates.
You can also create a WiX project from scratch by hand, as explained in the documentation.
It is easy to create a simple MSI installer, with explicit lists of files. You can also "harvest" .csproj outputs and some other Visual Studio project outputs, as well as directories, etc.
Most any editor, including Visual Studio Express, will help you edit WiX project and source files as XML. You might need to tell it where the relevant XML Schema files (.xsd) are to get content assistance. The zipped binary distribution of WiX might be simplest to use in such scenarios.
Windows Installer is very complex. The WiX Toolset allows full access to that complexity while attempting to kept it as simple as possible. Using WiX means learning at least some of Windows Installer.
Ultimately, no installer tool can look at your programming project and decide for you which files you want to ship and where to put them. So, no, you can't design and build an MSI simply by referencing a .csproj.