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.
Sources: U.S. Department of State, Treaty Trader & Treaty Investor Visas: https://travel.state.gov/content/travel/en/us-visas/employment/treaty-trader-investor-visa-e.html
9 FAM 402.9, Treaty Traders and Treaty Investors: https://fam.state.gov/fam/09FAM/09FAM040209.html