How to build a SoC computer?
It's certainly possible - the Raspberry Pi team were self-funded, and did the design themselves. However it is neither cheap nor easy to do it yourself.

1) Cost is much greater for small production volumes. Getting 1 SoC computer PCB made might cost you $100.

2) Not all the chips are available to the public, especially the more cutting-edge ones. The normal purchasing process for the Broadcom chip used in the Pi involves buying thousands. The "stacked RAM" process used isn't friendly to small volumes either.

3) If you think the Raspberry Pi doesn't have enough computing power, you may be misunderstanding your project (you haven't said what it is).