Originally Posted by
Mayfly
This will be my first tri, but I want to compete. I have a feeling I could finish in the top 15 of this race, by looking at the times.
Have you been doing many brick workouts? Your assessment may change after a couple of those. Or maybe you're just a much more gifted athlete than I am.
Regardless, if this is your first tri and you don't even have a road bike, get a decent, used (or cheaper new) drop-bar road bike and start with that. It'll be orders of magnitude faster than your mountain bike and if it's your only bike, it's nice to have a regular roadie for group rides, busier roads, more technical descents, etc.
Another thing to budget for--a wetsuit. There's not a single piece of bike gear that you can buy that will give you as much speed and time savings as a good wetsuit. Just figure that in when making your bike purchase.
There's also a tri forum here that I'd welcome you to take a look at. It's small, but the folks over there are good people. I'm the mod for that forum too.
Welcome to the addiction, and good luck!