iTunes doesen´t mind If u buy apps for a device u don´t own, the apps u buy saves/stores on your account, so even If u accidently erase or do something else that makes your iTunes freak out they will be saved on your account.
If u erase a certain app that u bought and trying to buy it again appstore will tell u that u already have purchased It before an u can download It for free again.
Yes u can buy apps specially made for the iPad and install them @ a time of your choice.