Generally, most potential buyers in today’s market determine how much capital to put down on a business through offering a down payment between one third and half of the purchase price. For many businesses, this means approximately one year's worth of adjusted cash flow.
However, please keep in mind that there are cases where these levels of down payment will not clear the existing debt in the business. It is in your interest that the company be transferred free and clean of debt, so take this into account when you calculate the amount that you will put forth.