RE: Advise on a .243
More important than the weight is going to be the type of bullet you use. You want something made for large game hunting, not varmint hunting. I would be looking at the 85 grn to 100 grn bullets made for this purpose. I'm not going to tell you to stick to a particular weight because you may find a different weight bullet shoots better in your gun. Try a few different weights from few different companies until you find one that works well in your gun.
There is really no such thing as "knock down" power for big game. It is more about what bullet does the most damage yet still yields good penitration. You will find most likely that your little .243 is probably more effective than your shotgun with the big heavy slug. Velocity, bullet weight and bullet construction all go hand in hand. A .243 is plenty to take a mature deer if you put the bullet in the right spot.
Paul