I'm a noob and in search of a road bike for a half ironman

I really know nothing about bikes so was hoping to find a bike I can learn with on marketplace. I'm just curious what i should and shouldn't be looking for. For example these seem like good deals:

The cervelo p2 SL for 350

or

The cervelo S1

Just looking for some guidance as I try to get into this new hobby. Currently a marathon runner and want to branch out! Thanks for any help!

https://preview.redd.it/cdx82i9zdboe1.jpg?width=720&format=pjpg&auto=webp&s=5817039903cd9b1674ec0b86bf15648031f31ac6

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https://preview.redd.it/p4t97i9zdboe1.jpg?width=720&format=pjpg&auto=webp&s=eff5cd15921fc4c5563a8a826ec0a4be8d86a024

https://preview.redd.it/wverei9zdboe1.jpg?width=720&format=pjpg&auto=webp&s=d8bed9c05d0f432b14cb9e2b407c5a6f8e8abe98

https://preview.redd.it/exw9ni9zdboe1.jpg?width=720&format=pjpg&auto=webp&s=5555734f1f5f55b56467dcb719b0981bf883ea70

https://preview.redd.it/t1zcuh9zdboe1.jpg?width=960&format=pjpg&auto=webp&s=37d19edc2c654136b8d8cd40a36d711d2669a83a

https://preview.redd.it/lwpbhk9zdboe1.jpg?width=960&format=pjpg&auto=webp&s=7a7355d153398d79e36b1cec20d6fad1f92e7896