Imagine that it is 1952. How would you write a letter to your congressman explaining why you think the Marshall Plan is a wise investment or why the Marshall Plan is effective?

To write an effective letter, try to list the economic and political benefits of the Marshall Plan to your congressman. Above all, try to explain how his support of the Marshall Plan will cement his place in American history as a proponent of European stability and American prosperity.
This second tactic would be more of a psychological approach; you're essentially highlighting what your congressman's support will mean for his own political advancement and legacy. Never underestimate the effectiveness of such an approach!
Describe how your congressman can use his support for the Marshall Plan to invigorate his own re-election campaign. Highlight his support for the expansion of American political influence through monetary/ capital investment and how this investment protects Europe against communist hegemony (dominance or rule). Remember that, in the 1950s, public suspicion against communism was at an all-time high. Senator Joe McCarthy capitalized on national fears about communism when he asserted that communists had taken over prominent positions in the country. Read about McCarthyism and the fear of communist rule after the post-war years.
If you like, you may be able to use some of the material from the link above in your letter.
Below are some of the economic and political benefits of the Marshall Plan (ones you will want to include in your letter).
You can state that the Marshall Plan is an effective tool in the Cold War conflict between the United States and Soviet Russia. Instead of committing the blood of soldiers to physical combat, the United States will be using monetary investment to protect Europe against communist hegemony and Soviet influence.
The Marshall Plan increases European productivity by easing production gridlock and curbing inflation. This will support Europe's increasing economic independence and bolster its political and social stability. Europe will also have the resources to strengthen its military defenses against Soviet encroachment. 
You can also explain how the Marshall Plan is an effective deterrent against an extreme form of nationalism (the exact kind that fostered Nazi-fascism and subjected entire populations of Jews, Christians, gypsies, the disabled, and homosexuals to the horrors of the Holocaust during World War Two). The Marshall Plan works to promote economic prosperity within an integrated Europe.
As a supporter of the Marshall Plan, you can argue that a strong European Union may just prevent future American involvement in armed conflicts. For example, if Europe is politically and economically powerful, it will cease to look towards the Soviet model for relief. Essentially, any economic prosperity built on the principles of capitalism will prevent Europe from capitulating towards failed communist theories about economic growth.
Since you do not state how long your letter needs to be, I leave you with some good links below to peruse (in case you need more information).
http://www.marshallfoundation.org/library/wp-content/uploads/sites/16/2014/05/The_Marshall_Plan_A_Strategy_that_Worked_000.pdf

https://www.marshallfoundation.org/marshall/the-marshall-plan/foreign-assistance-act-1948/

https://www.marshallfoundation.org/marshall/the-marshall-plan/history-marshall-plan/