I second mare milk replacer. Bottle feeding is a headache; however, finding a nurse mare is a) expensive b) hard during this time of year.
A nurse mare is expensive and often requires that it be sent back to its original home in foal. Which means you pay a stud fee in addition to housing this mare. The way they "train" nurse mares is to have their foals wear a blanket and train the foal to a bucket. Then when the mare is needed they take the mare and blanket. The put the blanket on the new foal and the mare recognizes the scent and does not make a fuss. To have a mare take on and nurse her foal and another mare is going to take a very special mare.
I say colostrum is the biggest concern. Otherwise, you risk sepsis and foal death due to infection because they lack an immune system.