bookstore app for android OS and iOS

I have some e-books, want to convert it to bookstore app. Bookstore is divided into no. of category say one category is JAVA, it is further divided as core java, advanced java etc. Now core java is divided into 4 sections and each section should be purchased separately. Integrate paypal for payment processing. Books contains question answers. So 1 section has 50 questions. Other sections are disabled till user purchases it. When the section is purchased, new content for that section should be downloaded from server. Server will have a database to store question answers. Both applications for iOS and Android OS. So you can develop using adobe flex. My budget for this is 300$-400$.