That's what I would suggest. Build your cage as large as you have space for, and in the mean time you can either divide it for your baby or get a smaller Reptarium for it to grow up in. You can always use the Reptarium for a portable outdoor basking cage later. Some screen and some plexi can work, but you may have problems with the cham seeing reflections of itself in the plastic. Also, chams don't "get" glass. They aren't rocket scientists. They can spend hours pacing and pawing at the surface trying to get through it. You could use the plexi on the back side against your wall, and use a type that is not completely clear or very shiny. It would protect the wall from overspray, keep a bit more humidity in, but not cause as much stress.
When you select your baby don't get one younger than 3 months. Really tiny babies are touchy and often they are sold much too young to handle stress of shipping, being in a shop or show, etc. Also, some percentage of most clutches are not destined to survive for congential reasons...and if you buy one too young you just might get one that will not survive for unknown reasons.