Install Front End server (OWA) with SBS 2003 backend?

We would like to use OWA for remote webmail to Exchange server installed from SBS 2003 standard edition.

Is it possible to use the same SBS 2003 Standard Ed. software to install Exchange Front End server on a Windows 2003 R2 standard server?

Our first attempt some time ago, we didnt notice an option to install FE server, just the full Exchange implementation.

Not sure if installing a new full Exchange in our domain, and trying to convert it to FE server will break existing Exchange or AD environment.

Or do we need to purchase/use another version Exchange 2003 or upgrade from our SBS 2003 standard to get the Front end install option.

Thanks in anticipation.