Just a couple notes regarding airguns.
1st you can make a really good backstop with an old electrical junction box and some duct seal. It will be heavy but will stop anything you throw at it.
2nd be aware that airguns that are spring powered are a totally different beast then firearms and require a completely different skill set to shoot accurately. If you try to apply firearm disciplines to them you will likely be frustrated, whether rifle or pistol. What they WILL teach you is trigger control, follow through and consistency. These skills will obviously carry over to firearms.
3rd C02 or PCP or SSP airguns can provide a more firearm like experience. Many C02 replicas will even work the slide. Be aware, however, these are WAY LOUDER then you might think they will be. Also I find C02 guns typically have horrendous triggers and accuracy that is sub par. PCP and SSP type guns usually are much much more accurate but lose out on the reality.
4th Airsoft actually does a good job of replicating firearms and can be a great drawn and fire/gun handling/shoot on the move type training tool. They are not generally all that accurate, although good ones are more accurate then you would think.
Just a little food for thought. Enjoy and BE SAFE.
Chris