I've got showers at work, but my experience is that changing clothes is usually sufficient. I sweat a lot on the way to work, but I don't think it's causing me to stink (no one has said so at any rate). I guess this varies from person to person, but I bet a change of clothes and sink bath will work for you. In the past, people have suggested joining a local gym and using their showers as another option.

Equipment: that's a nice bike. Do you have a good lock-up strategy? You might be at the "keep-it--inside" level, depending on your neighborhood. You could try a back pack before you make the switch to a rack. You'll probably be sweating lots either way, and a backpack works for a lot of people.

Fitness: It sounds like you've got this down, but you might consider a rest day after one of your 70+ mile days. Depending on your terrain, you might be able to take it easy enough on the commute to make it a rest day. In any event, you could probably handle it, but the group rides would probably pay bigger fitness dividends with rest the next day.