Hi Mirco,
your idea is good - you can use UART for GPRS module on TRXEB and have the wMBus Stack on the MSP430F5438A.
you'll have total of 256KB FLASH memory for your GPRS control application and wMBus stack as well.
Note that there might be different solutions for a data concentrator (DC), based on the complexity you need.
A simple DC can be done with the TRXEB+CC112x but if you need Ethernet connection, then you'll probably use some of our latest Cortex-A8 devices:
http://beagleboard.org/
The latest BeagleBone HW is a very powerful platform for Data collectors.
Regards,
Milen