Answer:
When Thoreau says <em>superfluous wealth</em> he refers to money that is not needed or there is more of it than enough and that with all that money can be bought just things that we do not need. Those things make us blind for what should be really important in life. As he goes on in the second sentence - we can have money, but we can not buy what our soul needs. Life can be experienced far more fully when living simply.
Spring would be the most inappropriate.
Spring is commonly used in literature to represent things like birth, new beginnings, growth, etc. Overall, Spring is used to show positive themes. To best represent a nearing death and other negative themes, an author should opt to use Winter or Autumn.