I used my iPad Mini 4 as a primary device for a while. Sold it because it got very slow and I wanted to cut my losses when the new model was announced. I used the Microsoft Foldable Keyboard but have also heard good things about the Logitech Keys-To-Go and Brydge 7.9. I am probably going to buy the 2019 model sometime before I go back to school—it's a great device for both traveling on trains and planes and studying.
Other must-have accessories are a portable charger, a wireless mouse for iPadOS, a stand, a game controller, an Apple Pencil, an Apple Pencil charger or a hub, a guitar interface if you make music, a case, a screen protector, and a bag.
Good apps are Soulver, Tweetbot, PDFelement, Luna VPN, Yoink, Spark, Wikipanion, GoodNotes, Opener, Drafts, Pythonista, Shortcuts, Launch Center Pro, Ulysses, Editorial, Photoship Mix, Procreate, and Kindle.