Well what I would do is take the tank and start growing out your aro in it, then move him to a temporary pond (rubbermaid). Take apart the tank and use the glass for the front of a 6'x4'x2' plywood tank. Which will give you enough room to house him for life at a decent price.
I would just take the tank, fill it use it for other fish for right now. Once you get a solid plan for how you are going to acquire your final tank then plan for about 1 - 1.5 years before that fish isn't going to fit any longer.