If you plan on staying in the hobby, go ahead and buy the computer.

PPM and PCM refers to the way that the signal is sent from the transmitter to the receiver. PPM is linear. YOur transmitter sends out x number of pulses in y time depending on how far a stick is moved. The receiver counts the pulses and moves the servo accordingly. PCM is digital and sends a string of 0's and 1's telling the receiver where to move the servo.