Mobile App For IOS&Android

1- I need an app for both platforms, a flashcard app for my students to study, I make the flashcards, users will download and look, that's all 2- App works like this: there will an online database (example: mssql), which consists of small sized images (.png), and my users will download them and my app shows them in the way I order those images 3- I will make small changes in the images from time to time (once in a week), when I make changes, user will be notified (example: push notification), and download only the new images, or delete the images that I deleted from my online database: so the app should be a mirror of my online database 4- App is mainly offline, so when there is no changes in my online db, there should be no operation between local device and online database 5- When images downloaded, will be stored in the local device, images can be searched, or rated, so there should be rate section like 1 star, 2 star, 3 star, etc. But all of these are local, specific for that user, so no user-to-user connection is required 6- User won't make any changes to my images, I will draw them, send them to online db, user will download them, that's all 7- There will be free user and pro user; free user will have %10 of images, total amount is 20 mb or so, free user will be pro by in-app purchase, then download rest of the images 8- User should be able to send me feedback of what do they think about images, and this should be different then app rating 9- a flashcard is composed of 2 images; one is front and other is back, total amount for a card is 50 kb for x size, 100 kb for 2x size, 200 kb for 3x size. All the cards have x, 2x, 3x size; and there will be about 2000 cards, so total sie of db I need is 2000x350 kb = 700 mb or so 10- I also need an interface to make changes in my online database without coding (admin panel etc) 11- Please tell me the cost of maintenance for keeping online database and API for download operations 12- I want to stress that this is a mainly offline app, I won't change my database very often, and I don't want my user to do lots of operations with online database when there is not any changes 13- I will provide images so stay tuned :) 14- Since app is mainly offline, and there are free/pro users, I need an authentication system and pro user should be able to use pro content while his/her device is offline 15- For all of developers that use firebase, I want to make things clear, when there is no change in my db and when my device is online, I want to use my local db, not firebase cache or firebase online db