“Distrust every enterprise that requires new clothes” is a paraphrase of what American author Henry David Thoreau (1817-1862) wrote in Walden: or Life in the Woods (1854):
 
“I say, beware of all enterprises that require new clothes, and not rather a new wearer of clothes. If there is not a new man, how can the new clothes be made to fit? If you have any enterprise before you, try it in your old clothes. All men want, not something to do with, but something to do, or rather something to be. Perhaps we should never procure a new suit, however ragged or dirty the old, until we have so conducted, so enterprised or sailed in some way, that we feel like new men in the old, and that to retain it would be like keeping new wine in old bottles. Our moulting season, like that of the fowls, must be a crisis in our lives.”
   
Thoreau is saying to always be yourself, but the expression is frequently used as reasoning against jobs that require new clothes (such as expensive suits).
 
 
Wikipedia: Henry David Thoreau
Henry David Thoreau (July 12, 1817 – May 6, 1862) was an American author, poet, philosopher, abolitionist, naturalist, tax resister, development critic, surveyor, and historian. A leading transcendentalist, Thoreau is best known for his book Walden, a reflection upon simple living in natural surroundings, and his essay Resistance to Civil Government (also known as Civil Disobedience), an argument for disobedience to an unjust state.
     
Wikiquote: Walden
Walden (1854) by Henry David Thoreau is one of the most famous non-fiction books written by an American author.
 
Chapter 1: Economy
I say, beware of all enterprises that require new clothes, and not rather a new wearer of clothes. If there is not a new man, how can the new clothes be made to fit? If you have any enterprise before you, try it in your old clothes. All men want, not something to do with, but something to do, or rather something to be. Perhaps we should never procure a new suit, however ragged or dirty the old, until we have so conducted, so enterprised or sailed in some way, that we feel like new men in the old, and that to retain it would be like keeping new wine in old bottles. Our moulting season, like that of the fowls, must be a crisis in our lives.
 
Google Books
September 1854, Graham’s Magazine, “Review of New Books,” pp. 298-299:
Walden: or Life in the Woods. By Henry D. Thoreau. Boston: Ticknor & Fields.
(...)
I say, beware of all enterprises that require new clothes, and not rather a new wearer of clothes. If there is not a new man, how can the new clothes be made to fit? If you have any enterprise before you, try it in your old clothes.
 
14 May 1922, Trenton (NJ) Sunday Times-Advertiser, “Words of Wise Men,” pt. 2, pg. 4, col. 2:
If you have any enterprise before you, try it in your new clothes.
   
23 May 1922, Ann Arbor (MI) Times News, pg. 4, col. 7:
A wise man says, “If you have any enterprise before you, try it in your new clothes.” But the chief enterprise of some of us is getting new clothes.—Arkansas Gazette.
 
Google Books
E. M. Forster
By Lionel Trilling
New York, NY: New Directions
1965
Pg. 9:
“Distrust every enterprise that requires new clothes” is the motto one of his characters inscribes over his wardrobe.
     
Google Books
Third and Possibly the Best 637 Best Things Anybody Ever Said
By Robert Byrne
New York, NY: Ballantine
1987, ©1986
Pg. ?:
60
Distrust any enterprise that requires new clothes.
Henry David Thoreau (1817-1862)
 
Google Books
1001 Greatest Things Ever Said about Massachusetts
By Patricia Harris and David Lyon
Guilford, CT: Lyons Press
2007
Pg. 388:
We should distrust any enterprise that requires new clothes. — Henry David Thoreau
 
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Venice Harrop
‏@Venice_Harrop68
Henry David Thoreau~ Distrust any enterprise that requires new clothes.
2:23 AM - 12 Dec 2015