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I have recently built a similar system with a GSM modem and GPS modem (connected to 2 UARTs), various SPI peripherals and USB HID interface to connect to a PC host for configuration and reporting.
We used a PIC24FJ64GB002 - it has the advantage of coming in a 28-Pin SPDIP package so it is easy to build a prototype and has Peripheral Pin Select for remapping digital peripherals to I/O pins which simplifies PCB layout. see the Microchip web page PIC24FJ64GB002
@Dinesh: Can u pls guide me as to where we can get the above mentioned PICs in India, particularly Chennai? Most electronics shops don't seem to have them in stock! Or is there any site online which can deliver me the PICs within two days??