Any tank you can get is a good start and a 500 sometime down the road is a good plan. I started w/ a 10 gallon in the early 1970's and it took a long time before I had a large tank. One of the big deals is making sure that you have a place that would support that weight. A 500 gallon tank will weight somewhere around 4000# which is more than some floor joist will want to support.
If it were my tank I'd skip the silver arowana and go w/ a black. The reason is that the Asian breeders have been breeding the same genetic defect in silvers over and over and over but so far they've not bred blacks to the same degree.
Were you to get a silver arowana there's a freakishly high likelihood that the fish would wind up w/ a droop eye problem on one side. Some will say it has to do w/ the fish always looking down for food or some other unusual excuse. None of that is true. It's a genetic fault associated w/ line breeding the same genetic stock for their prolific breeding habits and not the actual long term health of the spawn.
Regardless of which arowana you opt for note that you absolutely must have a way of securing the lid(s). I've kept arrowanas for at least 40 years and although they're beautiful and one of the only fish you'll see nearly identical examples of in jurassic era fossils... it's also true that they're coming out the top and anyone that's kept arowana for any length of time has arrived in the morning to see a dried up horseshoe shaped fish laying outside the tank and sometimes a broken lid.
