Adding Wi-Fi to a TCP/IP application that is already running with Ethernet
The modules that have the embedded TCP/IP stack all have a Cortex M3, M4 or similar processor to run the stack. You need a module without the processor, so you can use the stack on your existing processor. They are available, I'm using a Murata SN8000, partly because the drivers for Freescales Linux are available. These processor-less modules are cheaper and have better performance than the embedded processor versions.