There was a time when you could just have your iTunes library just sitting on your main computer drive but now with HD 1080p movies/tv shows that library will quickly swamp your start up disk.
Certainly most of your music/tv/movies collection ( assuming you download via the Apple Music Store) sits in the cloud on Apple servers etc but if you want to remain independent of an internet connection it seems now pretty critical to keep most of your iTunes library on an external drive.
One technique is to have two iTunes libraries. One library on your startup disk, which includes your music collection and iPhone apps too. The other library on an external drive is for HD TV Shows/movies.
But if you want to keep your large iTunes library on your start up disk the only cheap option is the MacMini with a 2GB Fusion drive. The New MacBook Pro does offer a 2TB option but at a very high price.
Or third option might be to get an external SSD drive, Samsung makes one and use it via USB-3 and run your computer OS or iTunes library from an external drive entirely.