Not sure where you are located, but around here donkeys are going for around $75.00, and as feed prices go up, I expect the prices will come down.
I've had three horses offered to me this past month for free.
One will be more protective than two because the bond with each other insread of with the sheep. Depending on the donkeys, they might be ok as a pair. If you have other animals in with them, they won't be lonely.
Male llamas make good guardian animals for sheep too.
Not much help, but I...