I have a little over 300 apps, many of which are games that I rarely play. Most apps are pretty standard in size (1-10 MB), but some, especially games, are hundreds of megabytes. One, The Dark Knight Rises, that I'll probably never play and should delete, is almost 2.5GB! So altogether, including the Android system (I think a little more than 1gb), cache (quite variable), the apps, and all their respective data, I use about 8.6 gigabytes. For a more realistic evaluation, take away TDKR, and that makes only a little more than 6 gigabytes used.
This leaves about 5 (7.5gb without TDKR) gigabytes free on my 16gb model. Yes, 8.6 used + 5 available doesn't equal 16, but that's what you get after all the formatting and whatever takes place. Don't let that part worry you. It is what it is! Anyway, this should give you an idea of how much space you'll have available after you install a boatload of apps you rarely use.
Nexus Media Importer lets you attach a USB flash drive with a USB OTG cable, and stream and/or copy media (music, videos, photos, etc.) and documents (pdf, text files, etc.) to the N7, without rooting your device. It works well, but has some limitations. Only media and some document files are recognized; some files, like pdf's, must be copied to the N7 to view - you can't view them straight from the flash drive like you can with videos and music; and you can't copy files from the N7 to the flash drive - it's read only from the flash drive to the N7. If these limitations don't work for you, you'll need to root your N7 and install an app like StickMount.