I agree with the other ladies... a HIM for a first tri is a whole different beast... than a Sprint or Oly.
Is there anyway you can do a sprint distance tri before making the decision? Maybe if you do one race in open water, you will get a feel for what you can handle.
I have to say though, I have seen very motivated people do amazing things. Some people are just naturals at the sport... and start out with HIM or an IM and do a great job.
Since I don't know your fitness level intimately, it's really hard to say what you can and can't do. Only you can do that. And if you can run a "test" with a sprint... to see what you are up for... that might be a good plan.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"