If you got the apps from the play store under your google account, they will all be available once you log in with your same google account. You can finds them under My Apps in the play store. While you can reinstall the apk from the external drive, it's simpler to reinstall from the play store directly; you'll get the latest version. It'll also give you a chance to clean house...I find a lot of extra apps I really don't use that take up space I can use for other things.
The only thing is to be sure you are on wifi when you do download all the apps again, unless you have no worries about your data use.
Are you looking for a backup similar to how the iPhone stores a complete image or just the data itself? What type of data? Pictures and such or application data?