Working with an unhealthy animal is a rich learning experience. The more you do it, the more comfortable you'll become and the more you'll develop a 'knack' for dealing with such animals. Adopting an adult is preferable to many people, as hatchlings and juveniles tend to be more delicate and skittish than adults. Many rescues have never been properly tamed and can be aggressive, frightened, skittish, etc. Many will become even more so once they become more healthy.
Rescues take a lot of special work and finances.
Many rescues will need to be seen by a qualified reptile veterinarian, and some will require multiple visits and treatment. Some may never become the pet you desire, They may be too ill to survive in the long run, or they may be too abused to learn to trust humans at all. It's sad, but it happens sometimes. Some people, naturally, want to raise a baby iguana, and babies are hard to come by in rescue and for adoption. Special needs of a rescue.