Come in and Talk about iguanas!
260 should be fine for an iguana, Make sure its on its side however, so it is longer instead of higher
They get put together fairly easy. You have to connect the frame, then the mesh goes over it. Depending on the temperment of your iguana this may not be a good idea becuase im sure it wouldnt take much to rip the mesh. A wired wall cage would be the best plan. *make sure the bottom isn't wire *
Some of the reptariums come with a plastic bottom some dont. Make sure you get it for it will save you alot of time cleaning.
This is the cheaper route, the safest way is to go with a wire cage. A huge kitten cage or Dog crate will work great -- keep those in mind.
As for what to do with your 20 gal. That is entirely up to yourself
There are many options for what you can do, and its not up to us to TELL you what to get.
Having said that though leo's would be a good idea, or a crestie would be nice too, if you fliped your tank over. So do your research and make sure its not an "impusle buy"
Hopes that helps
Chris