A green iguana, while very inexpensive to get is agreed in most circles to be an experienced herpers venture. It is not a cute little pet---it will change and just how much is yet to be seen. You can do everything right--spoil it until the cows come home and if you let you're guard down for an extended period of time while it's a baby---you will have major problems later when it's an adult.
Look on this website under IGUANAS (the forum) and read all the recent posts---including the photos of the stitches and bites and ask yourself if you really want something like this in your home.
If you really can't be swayed...here are the pros and cons for adult V baby:
CONS for getting a baby iguana:
1) an iguana can grow into a lizard you barely recognize. An adult iguana is a better idea because any rescue worth it's salt will fill you in the ig's temperament and history.
2) Sexing is virtually impossible so you might end up with a male. Male= UNEXPECTED behavioral problems later....this can happen all in a few seconds.
3) Baby Size is the worst possible preparation for what you will need later
4) Most are wild caught because they are so cheap to import....parasite problems. Vet bills.
PROS to getting an adult rescue:
1) Many health problems have been addressed and most at least identified.
2) Temperaments have been established.
3) Size is absolute--get one cage
4) Sometimes cages come with rescues
5)Also like Brett said you will be giving an Ig a home ( I would definately adopt if I wanted a green iguana--if you read anything on the net at all you see just how many homeless Igs there are).
Hope this helped....I still say buy a tegu

Think big and controlable with caging--it's a tough combo.