Welcome to the board. I've been looking for one of those older Eros' for a long time. Great bike.

Personally, I think you'd be better off on the main roads. As you said, your chances of getting wacked by someone not looking for a bike is greater. This is the only time I've ever had any issues really is when riding through neighborhoods.

For clothes, you can easily wear what you have for that length of commute. Just take it easy in the morning. Maybe put a rack on the bike and a rack trunk or pannier and put some athletic clothes in it for the ride home. This is what I do as well. Make sure and get something to wrap your right pants leg though. Something like a strip of velcro works fine. If not, you'll get some nice grease on your pants leg.

I recently picked up a set of Trek Interchange panniers for $100 and really like them. Easy on and off and can hold quite a bit. Most places have Trek dealers. Check their site.

Welcome to the club. You'll soon be asking all sort of stupid questions. :-) Most people here are very nice and willing to help. Before too long you'll be wondering why you haven't been doing this all along.