The locomotive is 29 inches long: https://mthtrains.com/20-3300-1
PS2 or PS3 both will run under DCS. So either one is fine. My Premier GS4 was originally equipped with PS1, but was converted to PS2 by its previous owner. I paid a little over $400 for the locomotive.
As far as a used factory PS2 locomotive it would go for a little more. Depending on how much use it has had I'd say about $600 on the high end. $700 seems like a bit much to me. Very few of the newer models hold a lot of value, especially when versions with better electronics come out.