Ameritrade Aims to Help Clients Invest: The TD Waterhouse buy propels its strategic shift toward guiding and advising investors.

Mergers and acquisitions have been a way of life at Ameritrade Holding Corp. The Omaha, Neb.-based discount brokerage, spurred by the belief that scale is the key to success in the online share-trading market, has bagged eight deals over the past five years, including its acquisition of National Discount Brokers Corp. for $154 million in 2001; its $1.3 billion merger with Datek Online Holdings Corp. in 2002; its Bidwell & Co. buy in 2003; and purchases of the online retail accounts of BrokerageAmerica, Mydiscountbroker.com, Investex Securities Group, and J.B. Oxford & Co.

In its biggest score, the company bought rival TD Waterhouse USA from parent TD Bank Financial …