Given that the character is a successful fashion model, it's probable that she is in her 20's, even if the script doesn't explicitly say that. Like you, I don't know it well, and only ever saw the Encores! concert some years ago.
I suppose that, if this is meant to strictly be a concert performance, LaChanze might give it a go, especially as her participation would create a lot of buzz for a new company starting out. I think people would be more forgiving of her being not quite right for the role if this is a "formal wear and music stands" event, rather than an actual full production.