Bike Suggestions

So, I've been having a lot of trouble finding advice on a bike. I've been frantically looking over craigslist and ebay, but without much luck. I would love something that can handle mild/moderate trails and fast enough to manage a triathlon (hybrid?). This will be my first triathlon, though I do have other race experience (marathon, 10k, etc...). Someday down the road I would like to invest into a nice tri bike, but at the moment I need something that can fill multiple roles (trails, commuting, etc)

 I will be training with a friend who has a Trek 7.2fx, and this will be her first triathlon as well. So for under $400-600, what should I be looking at?