I am currently married to a Brazilian girl and we are in the process
of getting her papers.  I am concerned though that the INS may view
our marriage with suspicion for a couple of reasons:

1. We got married in Las Vegas.
2. She is Brazilian.
3. We have no shared assets because she doesn't make any money
   so basically I take care of her financially.  She has a credit
   card and I give her spending money.  I pay for her school and
   everything else via the credit card.
4. We never had a church wedding.  Wasn't much point since all of 
   her family and friends would not have been able to come and for
   me its just a waste of money.

We do live together and have been since she moved from NY to be with me.
I guess the question is how much is enough to convince the INS and are
any of these things a serious red flag for them?

Based on my research, you have reason for concern.  According to
"Immigrant visas for spouses of US citizens" by Boyd F. Campbell,
Immigration Law Center, LLC (2002)
http://www.visaus.com/marriage.html, several of the facts you have
related are likely to attract the attention of the USCIS, the
successor agency to the INS.  In particular, marriage outside of your
respective ethnic groups is one issue.  An absence of evidence of
commingling of assets is another area of concern.  Third, you need to
be able to prove that you had a legally valid marriage; a legally
valid marriage need not have occurred in a church, but it must comply
with the laws of the State of Nevada.

I have provided you with a number of resources below that will give
you a good idea of what you can expect during your interviews.  The
problems you have identified do not appear to be insurmountable, but
you need to be prepared to address them in a credible way.  Documents
establishing that you are living together and you have been supporting
her financially will be important proof that yours is not a sham
marriage. Pictures of you together and with friends are also helpful. 
However, one source cautions not to overdo the documentation since
that can create suspicion.  I strongly encourage you to consult with a
qualified immigration attorney prior to attending an interview.

Sincerely,

Wonko

"Marriage Must Not Have Been Entered Into for Immigration Purpose"
Zhang & Associates, P.C. (2005)
http://www.hooyou.com/marriage/notimmigration.html

"Valid Marriage" Zhang & Associates, P.C. (2005) 
http://www.hooyou.com/marriage/valid.html

"A Suspect Marriage and Investigation of INS"  Zhang & Associates,
P.C. (2005) http://www.hooyou.com/marriage/suspect.html

"Immigration Korner" Compiled By Felicia Persaud, Caribbean News
(2006) http://www.caribbeannetnews.com/immigration.htm

"Green Cards Through Marriage" Law Offices of Carl Shusterman
http://shusterman.com/marriage.html

"How Do I Bring My Spouse (Husband or Wife) to Live in the United
States?" U. S. Citizenship and Immigration Services
http://shusterman.com/cgi-bin/ex-link.pl?uscis.gov/graphics/howdoi/spouselive.htm

"Marriage Interview" Sobolevsky & Associates LLP (2006)
http://www.sobolevsky.net/PracticeAreas/Marriage-Interview.asp

"Frequently Asked Questions" VisaPro.com (2006)
http://faq.visapro.com/Marriage-US-Citizen-FAQ4.asp This site notes
that unless you have been married for two years at the time of your
first interview, the USCIS will investigate your marriage again after
two years before granting a permanent green card to your spouse.

"US IMMIGRATION: INS is now USCIS" Pro Info ONE (2003)
http://www.visa2003.com/ins-uscis.htm