Well from what I have experenced, they are not very livley unless they are hunting for prey. Mainly just hover in one area untill something swims close enough for it to catch.
Really some gars are not very agresive. They are ambush hunters. And I never really took a fish's entertainment value to be how agresive it was.
I dont agree with keeping a fish just to able to watch it eat other fish. If however this is what you are looking for there are some better options out there, but they would all require a large 75+ gallon tank. Some of your south american cichlids will eat smaller fish when they get bigger, but are easy to maintain on pellet food.
I dont think you have fully taken into account of a fish that only eats live prey. One , it will get expenceive having to buy feeders constantly to keep the fish in good health. Two, what are you going to do if the LFS runs out of feeders, and will be unable to get more in for several weeks? A lot of your preditory fish have a high metabolisim, meaning that they need to eat almost constantly to maintain good health. I have heard lots of people say that they will buy sometype of live bearer to produce food for the fish, however the fry would take well over six months to reach a size that would be nutritinal for the preditory fish.
Just a few things to think about before you purchase.