Best of luck!
PS-You should be working with an agent who knows how this works to avoid suprises or it may not close.
If the seller is losing the home to foreclosure and it is already in the redemption period, they have no incentive to spend a single cent of their own money. If you are willing to offer enough to pay off the mortgage holder(s) and all the closing fees the seller shall incur (you can also roll your costs into the loan like Lenny mentioned), it is pretty straightforward.
The problem lies when you are making an offer that is less than what's owed. If that's the case, the seller would have to go back to the bank(s) and see if they will take less than what is owed. If the seller is very willing and cooperative, this can work out but can take anywhere from 2 weeks to 2 months to get an anwer. If the seller is unwilling and/or uncooperative, it is best not to waste your time.