Good morning James,
While a 640 score may be sufficient to qualify for FHA Insured financing with many Lenders, I have some more practical advice for you and your fiance. I strongly recommend you review your finances, especially your credit, and take your time to prepare for the homeownership experience. Buying a home and taking on a mortgage loan is a HUGE step. If your credit scores are less than 680, you really need to get better prepared so that you don't find yourself in trouble soon after buying your home.
Folks with poor credit and insufficient financial standing who purchased homes during the BOOM got easy mortgage money, but look at what happened after the fact. The mortgage is your responsibility; prepare yourself with strong credit, sufficient savings, and consistent, reliable income before you consider buying a home.
Sure, there are zillions of mortgage people who'll volunteer to help you with financing now. Based on real world experience of 22+ years in this industry, I'd suggest those mortgage people aren't offering valuable assistance, only leading you to a potential financial disaster.