The Louisiana Purchase Gold Dollars were intended to be a celebration of the 100th anniversary of the Louisiana Purchase. The authorizing legislation of June 28, 1902 gave the Secretary of the Treasury broad discretion in selecting the designs for the coin. Thomas Jefferson was chosen as one subject because he was President when the Louisiana Purchase was concluded. William McKinley was chosen for a second subject in part because of agitation by numismatist Farran Zerbe for a second variety, in part because the Louisiana Expo was authorized under McKinley's tenure, and in part because McKinley had been assassinated in 1901.
The 1903 LA Purchase/McKinley Gold Dollar has the distinction of being the second U.S. coin to bear the portrait of a "real" American (the first was the 1900 Lafayette Dollar, with conjoined busts of George Washingtonand the Marquis de Lafayette), and the first coin to bear the portrait of a martyred President (Lincoln didn't make it onto a coin until 1909).
Gem examples of this date are plentiful, as evidenced by the PCGS Population Report and the PCGS CoinFacts Condition Census (CC). The two best examples known of this type are a pair of PCGS MS68s, one of which was certified in 1999, and the other of which is a recent entry into the census. Finding a nice example of this design type should be a relatively easy task.