Any iPad is,currently, a step DOWN from any MacBook!
iPads, currently, are more of a companion/adjunct device.
If you have already been a MacBook/laptop user, then the iPad ( any iPad) will not be a replacement for any full blown, full OS class of MacBook/laptop.
iOS is a mobile, scaled down OS with no access to a file system.
There is no external mouse or trackpad support (iOS onboard keyboard has a trackpad-like feature).
I use my large ,12.9 inch screen iPad Pro as a replacement for a MacBook because I have never been a MacBook/laptop user (I am, definitely a desktop Mac user).
So, I have been using iPads as my laptop/creative digital tablet device replacements since the first iPad in 2010, and I am very used to iOS and the way iOS works and I am very adept at iOS and iPads.
I still own and use an iPad 1, 2, 3 and Pro models.
iOS is a very app-centric iOS that will require you to use a lot more apps for things you may have needed fewer apps to do the same things on a MacBook.
If you are looking to the large screen iPad Pro, and you want to store most of your work, locally, on an iPad, your only real options are the 128 GBs or 256 GBs storage models.
Make sure you get a model that still has a version that is close to the end of iOS 9's end of life (the last version of iOS 9 is 9.3.5)
IMSO, DO NOT UPGRADE THE IPAD PRO TO IOS 10.
iOS 10, currently doesn't offer anything really useful to the iPad Pro user and currently still contains a lot of bugs.
STAY AWAY FROM IOS 10, for now!
For any iTunes content you have, you can connect up an iPad to your MacBook to sync music, any other iTunes audio, iTunes movies/TV shows, home movies, your own other movie content, and photos.
For backups or other remote local storage, there is iTunes, Apple iCloud (they have a 50 GB plan for $.99/month, 200 GBs of storage for $3/month), DropBox, Amazon has unlimited data storage of all type for $5/month ($60/ year!),
for examples.
Other ways to augment iPad storage is to purchase special, mobile wireless devices like these.
There are a few special storage devices for mobile devices. There are portable hard drives for mobile devices made by Western Digital and Seagate.
http://www.wdc.com/en/products/products.aspx?id=1330
http://www.wdc.com/en/products/products.aspx?id=1660
http://www.seagate.com/consumer/stream/
There are special mobile USB flash drives made by SanDisk (now owned by Western Digital)
https://www.sandisk.com/home/mobile-device-storage/ixpand
https://www.sandisk.com/home/mobile-device-storage/connect-wireless-stick
Good Luck!