You can take over an existing business or start a new one. Investing in an existing business is acceptable for an E-2 visa; the E-2 does not require that investment be made in a new business. The main consideration is that the business must be operational, and the investment must be enough so that the business can continue operating. In both situations, you will have to prove that the investment is real and not marginal, and you have a controlling or significant ownership interest in the business.