Like stated before, if you're in the states skip the asian arowana idea. Its not worth the risk. There is a few guys at the moment that got busted for selling/trasporting/owning asian aros and they face 5 years and $250,000 in fines.
I have 2 arowanas right now (a silver and a black), and they are easy to care for. They do NOT require live foods, mine never get any and are thriving. They eat mainly shrimp, smelt, beefheart, pellets, earthworms, and random other things. Live food just poses more health risks than benefits, but is ok for a treat if they are properly conditioned.
If you can supply a 24" wide tank either will be happy for a very long time. Wider is always better though, and a 7 or 8 foot long tank is prefered.
Silvers can be found for about $20 in the states (~6") and blacks will be $150 and up.
There are also Aussie arowanas available (jardini and leichardti), but they are not as accepting to other tank mates. They actually get quite mean as they age.
Silvers and blacks are safe with anything that wont bully them, or fit in their mouth.
Also, it's a bad idea to try to get 2 silvers or blacks (or a combination of the 2) in the same tank. One will dominate and kill the other. If you have a large enough tank and can house 4 or 5 you can keep a group. Of course there is always and exeption to that rule.
The main thing to keep in mind with arowanas is to keep the water "clean" and with constant parameters, and they'll be fine.
They grow 1-2" per month if properly cared for (up to about ~16" IME)