So sorry to hear about your Sunburst. What a bummer.
I personally like to keep pairs or at least even numbers of males and females. They usually pair off and if there are only 3 someone is always left out.
I would suggest you either get 1 or 3 horses. Sunbursts and Mustangs are both the same species just different color variations, so you can go ahead and get either.
If you know what sex you have left you can request the opposite sex if you like.