The steam from a shower did NOTHING for my son's croup when he was little. Cold mist humidifier was the only thing that helped (aside from prescription medication). You can find them for under $30 and they will work fine.
You will get much use out of it over the years... I've never used a warm one, only cool mist. I even used it when I was pregnant with #2, because my sinuses were so stuffy. :)