First, buy Dr. King's Bible and Peace Prize Medal. Don't quibble, just pay the ransom and be done with it. These items should be put on display in a prominent, publicly accessible location in the Capitol building.
A proper monument requires more consideration. It should not include a likeness of Dr. King for a variety of reasons but primarily because his legacy is not his appearance but rather the lasting impact of his contributions. It should not include quotes, no matter how famous or obscure. Such a life cannot be distilled down into a half dozen easily chanted lines.
The monument must be simple, elegant, enduring and powerful.
The keystone of the monument is a black granite obelisk standing proud on a cylindrical base and situated at the southernmost edge of the monument grounds. The southern exposure of the base carries a simple inscription:
Martin Luther King, Jr
Born: January 15, 1929
Assassinated: April 4, 1968
To the north sweeping from east to west and centered at the obelisk's base is a semicircular expanse of granite bounded by a single ring of granite benches whose outer edges are defined by the arc swept by the shadow of the tip of the obelisk on 15th of January and whose inner edges are defined by the shadow on the 4th of April. A lawn extends beyond the benches to encompass the full arc of the obelisk's shadow on the shortest day of the year. The spaces between the benches mark off the hours as the obelisk casts it's shadow throughout the day. Water emanates from the base of the obelisk flowing across the black granite skirt forming a rippling pool that offers a reflection revealing character not visible from other vantage points. Near the top the obelisk is pierced by an aperture from south to north with the north opening fitted with a prism. The aperture and prism are designed such that a rainbow of color spreads across the water on the 28th of August and during those brief moments water roars from the obelisk forming a mist lifting the rainbow for all to see. Throughout the year the sweeping grey shadow and the outward flowing water define the spire as a starting point inspiring visitors to take from this place some small part of the peace and courage it commemorates.