Using a Visual Studio Express edition will work just as well as a full version. Valve announced a while ago now that C++ Visual Studio Express 2005 was an officially supported compiler for the Source SDK, which goes without saying that Express editions are more then capable tools for programming a mod (or even a non-commercial full game). The only difference is that the full Visual Studio comes with some extra tools, and a commercial license.
Actually you can code commercial software with the express edition, the tools are just really limited compare to the Team Suite Edition (wich I own bwahaha), anyway you are right saying that is more than enough to mod.