Paying for homeless dogs vet bill?

If there is an SPCA in your area, they will help you by giving you a discount rate or letting you know the cheapest vet in your area. She does need to be checked out as the scabs may not be from fleas. It sounds like to me that you may have just found a best friend. All a dog really needs is love and companionship. She will love you more than you can imagine. Take good care of her, she'll repay you for it.