The phrase 'one need only think' is correct and usable in written English.
It is typically used to suggest that a certain concept is self-evident and does not require further explanation. For example, you could say: "To understand the importance of voting, one need only think of the history of the civil rights movement."
One need only think of Julie Taymor's "Lion King" and of the Henson Foundation's International Puppet Festivals.
One need only think of R&B crooners like Chuck Willis or Rudy Ray Moore or visit the Sunday services led by Edgar Kendricks.
One need only think of the fate of Maimonides, the great medieval Jewish philosopher, whose compilation of Jewish law, the Mishneh Torah, was burnt as heretical when it first appeared, to realize how mild much American Jewish discord is.
One need only think about Lenin's notion of the Vanguard.
One need only think of all the diseases that need curing, the energy sources that need finding and refining, the as-yet-undiscovered technologies needed to protect the U.S. and its allies against manifold threats to its national security.
One need only think of Jesse Owens's four gold medals in Berlin in 1936, shattering Hitler's plan to use his Olympics to demonstrate the superiority of the Aryan "master race" or Muhammad Ali's opposition to the Vietnam War by refusing the draft.
"They need only think: Good cake".
One need not only think here of serious late-onset disorders.
Younger ones typically need less water than older ones, while newly planted ones need only small amount of water.
Conversely, some celebrity casting has worked very well; one need think only of Linda Ronstadt in "The Pirates of Penzance" or Reba McEntire in "Annie Get Your Gun".
One need think only of the recently concluded story of the Tsarnaev brothers, fifteen hundred miles from Miami, to recognize this truth: the casual passage from dormitory weed to mass murder, the citywide lockdown that begins after the terrorists find themselves, completely by chance, in a 7-Eleven that some other thugs have casually decided to rob.
