I work for the mortgage division of a local bank. We originate and fund loans for 1-4 unit properties with the clear intent of selling the loan on the secondary market just like most major mortgage companies and banks. While there can be some variances (such as FICO scores and minimum down payments) between us due to individual arrangements we have with the major purchasers of these loans, most of the guidelines we apply are the same.
Since you have asked regarding the down payment required for an "investment" I assume that you do not intend to occupy any of the units of this property as your own residence. Because the industry believes that "non-owner occupied" loans are riskier, the minimum down payment is higher that if it was going to be occupied by you. Generally, this means at least 25% will be the lowest down payment that we would require for a multi-unit property that has 2, 3 or 4 units. Single unit properties can be purchased with as little as 20% down (or as little as 15% down if secondary financing can be located). One large exception to these numbers are properties sold under Fannie Mae's Home Path program, which is one way that the Federal National Mortgage Association tries to market their repo'd properties. Since there are other rules that apply for investor purchasers like yourself, be sure to get thoroughly pre-qualified by a knowledgeable lender early in your search.