I also think the PIC16 or 18 is plenty fast for the application. But the servo's and speed controllers are pretty picky about their signals. They need to be at regular intervals in order for the servo's and speed controllers to work smoothly. Otherwise their operation can be erratic. While you could measure the receiver with pulsin, I would stick to interrupts for the servo and speed controller. The PIC18F4431 has hardware that allows for R/C pwm frequencies. But it could be done with pretty much any PIC using spwm. I would probably use a PIC18 rather than a PIC16 because it opens up a couple of possibilities, but both are powerful enough.