I suggest you visit the Russia, Ukraine and Belarus regional forum as the Warsaw option is discused quite a lot already.
With that said, NVC will not be sending you any kind of welcome package nor will they be requesting anything from you or your fiancee. The extent of NVCs communication with you will be limited to an email informing you of your new case number and informing you that the petition has been forwarded to the assigned Consulate.
The K1 is 100% applicant driven so don't expect any assistance from the Consulate other than a set of instructions to be followed. All required documents are hand carried by the applicant and given to the consular on the day of the interview.