What is the effect of the first sentence of "Pride and Prejudice" and how would it change if "must be in want" would be substituted for "must want"?

"It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife." 

The sentence introduces one of the main themes of the plot, the pursuit of rich, single men. It establishes the prevailing social concept of the time of a "good marriage", and the second part of the sentence makes one realise that the reverse is also true: Single young women at the time had little other choice in life than to marry as high up in status and wealth as possible.

If we change the second part of the sentence to "must want a wife", the meaning changes. "In want of" suggests a need for wholeness, the necessity to be complete, which is only possible with a companion, not just with money. A simple "want" however, indicates possession rather than companionship. Here, a wife would simply be an accessoire to complete the man's exterior wealth. Ultimately, one could argue the two phrases are about interiority vs exteriority.