Help me figure out if I am doing the right tri bike purchase

Hi all. I'm about to pull the trigger on a triathlon bike, and I just want to make sure that I'm not totally out of my mind here with my decision making, since this will all involve a lot of cash.

I'm a 33M about to get into triathlons "seriously" - although I have have no road/tri biking experience. I have never entered a bicycle road race or triathlon, ever. I am, however, very comfortable on my mountain bike, and climb pretty well. I've been a pure marathoner , and averaged 60-70 miles per week of running for most of the past 2 years, and I usually can place smaller local running road races from 5k-half marathon (18:40ish 5k PR, 1:25 HM, 3:16M PR). I've started swimming a lot as well - I'm in the pool 5 days a week as well now. In the past 10 weeks, I have cut back a lot on the running and moved toward cycling, with 6-7 days per week on the indoor bike, riding anywhere from 45-90 minutes at a time, fairly hard.

I'm about to purchase a Cervelo P2C that the LBS has in stock. They would of course, make sure it fits me perfectly before I purchase it. It'll cost $2500 before tax, give and take. I'll also need to get everything else along with it - clipless pedals (which I've never used), clothes, trainer, car rack, the works. I figure I will be dropping $3500 by the time I'm done with it all, after taxes.

My rationale is that since I anticipate being a fairly serious endurance athlete, it seems reasonable for me to just get a bike that I won't ever outgrow now, despite the serious cash outlay, especially since my mid-range goal in the next 2 years is to race a half-ironman. My more rational side tells me since I literally have zero road biking experience, I should start with a lesser bike (like a entry level Jamis.)

Any thoughts on how this road bike rookie should approach this Cervelo vs lesser bike purchase?