I assume this is your first Tri? Like so many before you, you may find it is not your cup of tea. Spending a lot of money on a bike, especially a Tri bike, is not a good idea.
If you have any bike at all of decent quality I would suggest that you use that. A MTN bike with smooth tires would be fine. Regardless, I would not recommend clipless shoes. T1 is tough for the experienced, for the first timer it can be really bad. You do not want to be screwing around with shoes that you are not comfortable in yet. Get a set of toe clips.
The object of a first time Tri should be to finish, not go fast. Going fast is the best way to not finish for one reason or another - kicked in the face during the swim, wreck on the bike, blow up on the run, plus all the problems that can shut you down in T1 and T2. You will be amazed at the number of people with $3000+ bikes that do not clear T1 for whatever reason, or clear it late.
Lastly, I suggest that you head over to Runnersworlds web site and check out the Tri section of their forums. This question comes up quite often over there, and there are some very good explinations and answers.
And good luck. It is nice to see another person jump into the realm of being a real athlete.