Visual Studio executes MSBuild automatically for projects it supports.
If you right click on a project and unload it, you can then edit it in Visual Studio. Reload (right click on project again), force a (re)build to test your changes. An alternative is to edit the project file in an external editor and Visual Studio will detect saves and offer to reload the project for you.
Sounds like you're on the right track, and if you are considering writing Targets or custom MSBuild Tasks, take the time to separate them from your current project so that you can re-use them. Don't re-invent the wheel though, the two main complementary MSBuild projects are MSBuild Community Tasks and MSBuild Extension Pack.
Update: Judging from your comment on Mitch's answer, you might also want to consider adding a new Configuration element or custom properties to a project. A new MSBuild Configuration (something other than the default Debug/Release) could run unit tests, build documentation, or whatever you want automated. A custom MSBuild property would allow you to use normal Debug/Release Configuration and extend it to automate more of your build process, just depends on what you want. Either approach could also be driven from the command line.