David
I would start looking at the K20DX256x7 (as on the TWR-K20D72M)
- USB
- 2 SPIs
- 6 UARTs
There are a number of alternative pin mux options for these so you should have no problems finding the combination that you need
- 72MHz
- 100 pin LQFP
Then you could optimise on the package if needed (there are variations with less pins) and possibly on the peripherals (such as the K21 or K22) or choose parts with mort or less FLASH/RAM, max frequency or FPU.
Regards
Mark