Did you say she is 18 months old?  If so, then she's still a baby and should not be considered for guardian work.   Donkeys aren't really fully grown until around 4 yrs old.  That's when they get all their adult teeth.  They are slower to mature than horses.  Donkeys are very herd oriented.  They may do fine with a horse, but really prefer another donkey.  They will not bond socially with sheep, so it would be very lonely for her without another of her kind.  Also as stated before, she is very young and without older herd members around, she has no one to teach her social skills still needed at her age that she learns from other donkeys.  Just something else to think about.  
BTW, I've never had to sedate a donkey to load on a trailer.  It just takes patience and knowledge of the breed characteristics.  Once they've been on one time, they usually jump in within a minute or two.  I currently have 26.