What wheels on the P3? You probably have a couple pairs (race and training?). Whatever drivetrain works with those wheels.
Remember fit. You have exceptionally long torso or legs for your height? If not then you can start perusing shops, online place, etc. I'd look for machines with "race fit" head tube lengths (as long as they fit you properly) so you can get the bars low enough. As a tri guy with an aero bike you probably can handle a reasonably aggressive road position right away.
Longer/lower stems make the front end more stable at speed. Try and get a fit so you can use a 110-130 mm stem. I think a 120-130 mm stem is ideal for most riders. Taller riders probably work better with 130 mm stems.
Have a good, solid cockpit (bars, stem, shifters, fork, reasonably stiff front triangle on frame), right saddle (for you), and you'll be fine. You can probably get a used bike from someone, and if you do that, you may be able to get it sans wheels and maybe sans some other stuff that you don't need.
I'm riding the best bike I've had pretty much ever. It's not light, it's not sexy tight, but it works really well. It fits, it weighs 1450 grams (frame only), has a 450 gram fork, weighs 17.5 lbs with very light race wheels on it, and it totally rocks.
73 head tube angle (standard). A super long wheelbase (1010 mm? It's so long I had to move my front roller forward 3 notches compared to my track bike - 3 notches is about 3 inches, maybe 7-8 cm).
Stiff front end so it turns really well, I can put that front wheel anywhere I want.
For components? Get stuff that works the same as the higher end stuff (like 105, since the shifters work the same way as DA, but not Tiagra). Don't worry about level, you can upgrade later or when you decide you think there's a difference. 105, sure, whatever. If you have Shimano stuff, you may think about SRAM.
cdr