The thread you've replied to is over 3 years old, which will affect the number of people who see your post. Best to ask this in your own thread, or ask a moderator to move it for you.
But the answers are:
There is more than one cloud service, and they have different limits.. If yo don't want to pay to have all your stuff in one place, you can spread it out a bit. Also, keep an eye on promotions. I got 50GB of space on Box during one of their promotions.
DropBox and the other cloud services will only open (for viewing) the more standard file types. To actually edit them you'll need apps for each type of file you want to work with. An Office 365 subscription will let you use the MS Word, Excel, and PowerPoint iOS apps to edit documents. Without the subscription these apps are view only.
Since you just bough your iPad Air, you should have free access to Apple's office apps: Pages, Numbers, and Keynote. Check them out in the App Store. They will import and export to MS Office. Most people who only need occasional compatibility with MS Office find they work fine.
Apps have their own file space. When you use Open In to move a file from DropBox to another app, it makes a copy. This is about to change in iOS 8. You came at a good time.
If you want your music files in the Music app you will have to use iTunes on a computer. Download it from Apple's site, then import your music. Once you do that you can connect your iPad and sync.
You can learn a lot more by downloading the iBook app, going to the iBook Store inside the app, and getting the free iPad User Guide. Make sure you get iOS 7 version. There is also a PDF version available on Apple's site, and an online version. I don't have the links handy at the moment, but keep an eye on the signatures of the moderators. Most have the links there.