how much knock down power will 100 grain 243 have at 300 yards?

If you hit them properly it will work just fine. Several years ago I shot a 130 class whitetail at 330 yards. I shot him just before dark as he went into a small thicket in an alfalfa field. I had my scope on where he would come out. He stepped out and collapsed dead. With a 100 grain bullet to his heart.

Enough to take down a whitetail . I used to shoot nothing but a 243 and it was in a Browning automatic . I took many shots like that and seldom did I neede a second shot . If you just want it for whitetail and below it is an excellent gun , not very expensive and you can load it with a 75 or 100 grain and it will do the trick.

at 300 yards the 243 using factory loaded 100 grain bullets will deliver a little over 1000 ft-lbs of energy.

That's roughly what the 30-30 has at a bit over 100 yards.

Plenty of power at 300 yards to handle a whitetail deer

Hornady's 100 gr. BTSP round, at 300 yards, is traveling along at 2,299 feet per second, and still packs 1,173 foot pounds of energy.

http://www.hornady.com/store/243-Win-100-gr-BTSP/