Arduino USB host + RS232
1) Which Arduino board is the best fit? 2) Is it an option to use Chinese boards for the sake of cost optimization? 3) Do I need separate RS232 and USB shield or it's ok to leverage built in USB and data pins?


When communicating with Android you need to make sure that there is software available for doing the communication. That's really down to the application more than the operating system, and because of that it can be quite restricted what boards will work and what won't.

Personally I would go for either an ATMega32U4 based board which provides a standard CDC/ACM interface. This also has the advantage that the (often single) UART on the board is not used by the USB communication.

Then you need to convert the UART into RS232 using (for example) a MAX232 board.