If it were my tank, I'd go for one high quality asian arowana. As someone else mentioned, the cost of food, electricity, maintenance etc etc is going to be pretty much the same regardless of if you get a $20 silver or a $2k asian, it is only the initial cost of the fish that differs, and IMO if you're going to spend thousands running a big tank to house a fish of that size, then spend a few bucks more to start with and make sure you're going to get a fish that will be nice to look at over the next ten years.
Asian aro + some mid-swimmers like silver dollars, big barbs, cichlids etc, and some bottom dwellers like plecs, bichirs, rays or loaches and you'll have a very enjoyable tank.
