I'd consider a .357mag with a 6 inch or longer barrel the bare minimum and that would be loaded with at least a full power Magnum load carrying an 158gr bullet if not a 180 (if the gun shoots one better than the other, go with the most accurate one, if it's close enough to even, go with the heavier bullet). This would be for ranges out to about 50 yards unless you know for certain that you can make the shot EVERY time further out.
Personally, I think a .41Mag or a .44Mag (with a 6 inch barrel or longer) would be better and still the ranges would be limited to 50 or less unless I knew for a fact I could shoot it well enough out to 75 yards or so to insure a clean hit and kill. If we are talking about a Ruger (ie., the Redhawk or a Blackhawk on the older big frame) or Dan Wesson, you can throw in the .45 Colt as an option because they will stand up to the hot loaded '+P+' .45 Colt loads like those put out by CorBon or Buffalo Bore. There's a lot to be said about throwing a half an ounce or more of lead at Mach 1+ and any of the 3 big bore rounds I mentioned can do it with the proper gun, load and enough barrel length. Things tend to fall down pretty quick if you put that much lead in the right spot.
Scoping the gun is up to you. I took the 2xEER that was on my 8 inch Dan Wesson barrel shroud off because I was more comfortable without it. Shooting a scoped hand gun takes a bit of getting used to and I found it more frustrating than helpful. I wasn't shooting it far enough to need a scope, in my opinion, so it was just added weight to hold up anyhow.
My big reason for suggesting the longer barrel lengths is two fold. One, the longer the barrel is, the longer the sight plane is. The greater the sight plane, the shot can be made more accurately. Secondly, you get a increase in velocity with the longer barrel lengths. Higher the velocity is, the flatter the trajectory is. The flatter the trajectory is, the longer the range is that you can shoot accurately.