A: You can buy an existing business. E-2 is not limited to startups. What matters is whether the investment and enterprise meet the E-2 requirements, including substantial investment, lawful source and control of the funds, a real operating business, and your ability to develop and direct it.
Source: 9 FAM 402.9: https://fam.state.gov/fam/09FAM/09FAM040209.html