you could just use to original letter of intent, just update the date on it.
will also need any police certificates, birth certificate, divorce certificates (if applicable), 2 US passport style photos, and any additional evidence of the relationship (may or may not be asked for, but good to have. like photos and stuff)
and don't forget the evidence for the I-134.
tax return transcripts, W2s, employment authorization letter, etc.
your embassy should send you some kind of letter telling you exactly what you need to bring to the interview once they receive your case from the NVC, as every embassy is different. but those things are pretty standard.