If Audi (and Porsche) participate, it will likely be done through a partnership or a purchase into an existing team.
Building a factory team from scratch would take 5 years to get all of its R&D resources up and running, let alone have a car good enough to compete for wins and titles.
It used to be a lot easier, but F1 has never been as resource and knowledge intensive as it is today. Even with the budget cap. You cannot quickly acquire this knowledge and infrastructure if you start from scratch.
However, starting with the takeover of an exciting team can ensure this fast path.
Buying an existing team is far less risky and more likely to result in a return on investment (in terms of success-related marketing exposure) than starting from scratch. Even if it costs half a billion to buy a team.