Convert browser based access to website to an App

Write an Android App to replace existing functionality of a user logging onto a website with an Android browser to communicate with a proprietary modem with integrated GPS. Website version sample is here: [login to view URL] App has 1 main page then 11 pages leading from main page. The new application is to have approximately the same look and feel as the original website but will be an Android app instead of a website accessed with a browser on the Android machine. The old browser system gets information from text files. The new application will interrogate the modem directly via Bluetooth. The Bluetooth connectivity component of the application has already been tested and confirmed as working in principal though only with one command. The application will retrieve some information from the modem: a) Text messages stored in the modem. b) GPS information from the modem's integral GPS. The application will send some information to the modem: a) Messages created from filling in the boxes on each page of the application and pressing the Submit button. b) Acknowledgement of reading incoming text messages (maybe). Checksums must be calculated and added to the end of the message. Formula for checksum is available and commands to modem are simple text commands, though system must communicate using Bluetooth in real time to check for new messages and to get status every few seconds. Application is to be written in an open source, or free or cheap development platform so that the development platform and source code can be installed by the developer remotely on a Windows PC supplied by the client. The application must be able to be modified by the client and recompiled on this machine for installation onto the Android. Application should be written for ease of modification - variables are to be used for items such as Copyright owner, client, etc. Application is for private use and will be installed as an APK, not from Google Play