either a 4 or a 5 (fat or very fat) depending on how heart shaped. (scoring goes from 1-5, 3 being good). Have a look on the D&H website for a condition scoring pdf...
Depends on the breeding. My mare even when she is superfit for eventing she has a heart shaped bum and her foal is the same. You need to look at the overall picture ie including the belly.
my appy x cob is a bit of a porker at the moment however she is in a 42inch girth now compared to the 54inch girth she was in when i got her (really REALLY fat) and she has always had a round bum what ever weight she is