I would have an I-134 filled out and ready also. The K-1 visa is a Non-immigrant visa and according to the Foreign Affairs Manual which the Embassy Consular office is instructed to use, they are not authorized to use the I-864 for fiance(e) visas.
I-864 is used for immigrant visas, because they directly result in a green card being issued.
A K-1 visa holder will file an I-864 after marriage, and when adjusting status.
QUOTE
9 FAM 40.41 N4.6-3 Use of Form I-134, Affidavit of Support Under INA 213A
(CT:VISA-823; 07-14-2006)
a. With the required use of Form I-864, Affidavit of Support Under Section 213A of the Act, under INA 212(a)(4)© and INA 213A for so many classes of immigrants, the use of Form I-134, Affidavit of Support, has been reduced considerably. Nevertheless, there still are circumstances when Form I-134 will be beneficial. This affidavit, submitted by the applicant at your request, is not legally binding on the sponsor and should not be accorded the same weight as Form I-864. Form I-134 should be given consideration as one form of evidence, however, in conjunction with the other forms of evidence mentioned below.
b. If any of the following applicants need an Affidavit of Support to meet the public charge requirement, they must use Form I-134, as they are not authorized to use Form I-864:
(1) The self-petitioning spouse of a deceased U.S. citizen, and any children there from (see INA 204(a)(1)(A)(ii));
(2) The self-petitioning spouse of a U.S. citizen, and any children there from, who has been battered by or subjected to extreme cruelty
perpetrated by the spouse (see INA 204(a)(1)(A)(iii) and (iv));
(3) Returning resident aliens;
(4) Diversity visa applicants; and
(5) Fiancé(e)s.
U.S. Department of State Foreign Affairs Manual Volume 9 - Visas 9 FAM 40.41 Notes Page 14 of 35
hppt://foia.state.gov/masterdocs/09fam/0940041N.pdf
Financial evidence recommended is 1-3 years past returns, letter from employer stating income amount, nature of job, and temp/perm, and past 6 months pay stubs helps.
Another thing you can do is have a co-sponsor for the K-1 visa, they can provide you with I-134 and financial evidence, and when you do adjust status, you will be able to use your 2006 taxes and stronger support evidence.