NEW YORK/FRANKFURT (Reuters) - Warren Buffett's Berkshire Hathaway Inc (>> Berkshire Hathaway Inc.) said on Friday it had agreed to buy German motorcycle apparel and accessories retailer Detlev Louis Motorrad-Vertriebs GmbH for a little more than 400 million euros (297 million pounds).

The purchase enables Buffett, known for his optimism about the United States and the investment opportunities it offers, to expand in Europe despite its economic problems.

Ute Louis, the widow of company founder Detlev Louis, had approached Berkshire about a possible transaction and sold the Hamburg-based company to a Berkshire unit, according to the law firm Beiten Burkhardt, which advised on the purchase. Antitrust approval is required, the law firm said.

Buffett's assistant, Debbie Bosanek, confirmed the purchase price, which was earlier reported by the Financial Times.

Buffett told that newspaper that the purchase was a "door opener," and that "the U.S. is my first love, but I see terrific possibilities for us in Europe."

Most of Berkshire's more than 80 business units operate mainly in the United States, though the Omaha, Nebraska-based company has invested in European companies such as German reinsurer Munich Re (>> Muenchener Rueckversicherungs-Ges. AG) and French drugmaker Sanofi SA (>> Sanofi).

Detlev Louis has annual sales of 270 million euros, employs more than 1,500 people, has more than 70 outlets in Germany and Austria, and serves 25 countries through online stores, its law firm said.

The purchase fits Buffett's common practice of buying entire businesses and then leaving existing management in place.

Buffett has done this outside the United States. In 2006, Berkshire paid $4 billion for most of Israel's IMC International Metalworking Cos. He paid $2.05 billion for the rest in 2013.

The Detlev Louis purchase also expands Berkshire's presence in transportation-related industries.

Berkshire owns the Geico auto insurance business and agreed in October to buy the large U.S. auto retailer Van Tuyl Group.

Buffett is also no stranger to motorcycles. In 2009, Berkshire bought $300 million of debt with a 15 percent interest rate from Harley Davidson Inc (>> Harley-Davidson Inc).

Still, Detlev Louis remains a small acquisition for Berkshire, whose $62.38 billion cash hoard at the end of September 2014 left Buffett with the power to make one or more large acquisitions, which he calls "elephants."

Buffett told the Financial Times that Europe was fertile ground for such purchases despite its problems.

(Reporting by Jonathan Stempel and Ludwig Burger; Editing by David Holmes and Jonathan Oatis)

By Jonathan Stempel and Ludwig Burger