Hi
A start would be something with a 8x3 footprint, height is not so important with Arowana and Stingrays.
Excellent filtration - large sump. I would be incorporating K1 media in the sump, as stingrays place a huge amount of strain on biological filtration.
Arowana - unless you want to spend big $, go for Australian Saratoga - I'd recommended Leichardti - IMO look nicer and are generally much more easy going than Jardini - much easier to keep a group of them, and play nicer with tank mates.
Silver arowana pop up from time to time, but are not easy to get (recently seen Black Arowana too)
Seems to be a fair few small Asian Arowana becoming available - expect to pay 2.5K+
Stingrays are always available locally these days - Pup Motoro Rays usually start at about $250, considerably more for designer hybrid rays. Put up a WTB someone locally should be able to help.
There really is no secret with keeping tankbusters (rays and Arowana) - start out with the largest possible tank that you can accommodate into your life and budget (I wouldn't bother if that happens to be a tank with a smaller footprint than an 8x3). Excellent filtration - generally speaking that tends to involve a sump or I have found above tank trickle filters work great also (but they don't provide you with anywhere to hide equipment (particularly heating elements), or the opportunity to increase the systems total volume). Plenty of good quality food.
People generally become unstuck with planning to 'upgrade' into the future and not starting out with over the top filtration.
Tankbuster/Stingray keeping in Australia - Big budget... Big dreams.
Alex