Authenticated SMTP to relay outbound only

I have a Postfix mail server running on our intranet FreeBSD server. The application is very simple - it acts as an open relay for email pages from internal servers and emails generated by MediaWiki, Bugzilla, and Zabbix on the host.

So, open relay on the inside, relay by allowed IP to our ISP on the outside. This s a closed, firewalled, network. We have IPS at the gateway.

We are moving to Google Apps. I have to set up authenticated SMTP with a named account at Google in order to relay. I want to keep this as a simple as possible. I will only be using a single account at Google, and the message volume is less than 300 a day. I only need to configure a single username/password to do the SSL (with TLS) connection to Google.

I have read several tutorials on postfix with OpenSSL, SASL2, &ct. I am not familiar enough with all of the terminology to understand what I need. Most of these tutorials include lots of features and packages which are of no interest to me.

Could someone with knowledge in these areas steer me in the right direction please? I'd be most grateful.