So lets put some perspective on this.
Silver arowana max adult size 47 inches! can be obtained in as little as 3 years! typical size in appropriate aquarium min size would need to be double length and 1.5x width that's a minimum and they reach 36-42" regularly so 8'x4' would be an absolute minimum and not ideal at all.
Motoro stingray 36" max adult size and regularly hits 24" in aquarium slower growing but still pretty big fish oh sizes are in diameter! rays need a different set up instead of 2x and 1.5x max size you need to cube max diameter 3x so a 6'x6' minimum tank size for a ray, not ideal at all. That wont do for both ray and aro so you get answers like 10'x6' with a depth of somewhere in the 30"-40" zone, better but really? These die in the hands of seasoned keepers all the time. Not fish for Noobs as you put it.
These fish are awesome! I want rays and aro myself. I have had 2000+ gallons of salt water in my house at 1 time running in one huge system ive kept hard to keep salt and fresh water species alive , Ive propagated all kinds of corals, Ive bred clowns, Africa cichlids, hatched shark eggs and cared for those smaller species long term. I could set up a huge tank and keep rays and aro and I still may loose them based off of time or fragility or a million other reasons.
Stick with your plan get the size tank you can put in the spot you have and get some appropriate sized and hardy species that you'd like. Keep them long term and see what the maintenance is like. Try keeping your water pristine less the 20 ppm nitrate no ammonia no nitrite. Ph no higher then 7.2 long term. Then double you efforts and you will be ready for stingrays and arowana. I'm not. I know it sounds kinda harsh but this is a life you hold in your hands is it fair to give them the minimum?