I know you decided but I'll add my $0.2. I wouldn't advise getting the tegu if you can't build it a bigger cage then 6'x3'x2' (LxWxH) as that is the absolute bare minimum. I can't see it being too expensive, 3 sheets of 8'x4'x1/2" plywood (LxWxT) is too much. They are $18 a sheet here in Canada. Then you can either buy some lino sheets, a 6'x4' sheet is I believe $28. For sealing or buy a bunch of polyurathane.
So far for one of my 8'x4'x2' cages its only cost me $79.35 CAD which is like $61.68 USD. I've bought basically everything I need aside from the plywood, corrugated plastic, and the plexi-glass doors. PM for a complete list.
That cage is already REALLY tight for a tegu that size though. If your willing get a pane of glass for the top of the tank and silicone it onto the black rim then lay the tank on its side and cut/smash the side sticking up that cage would work for a bit longer. 5'x2'x1.5' isn't great but its better then 1.5' wide. Hope that made sense! lol
If your interested in buying a tegu I HIGHLY recommend purchasing a tegu from Bobby Hill at Varnyard Herps Inc. He sells 3 types of tegu, the Argentine B&W, Argentine Red, and the Extreme Giant (Chacoan Giant). Ranging from I believe $200 to $300. After the passing of Bert Langerwerf, Bobby has become the biggest breeder of tegus and has the best customer service I've ever heard of. Even if you don't own a tegu he will help you! They will also be for sale around June/July so you can save up.
Good luck