You may find it pretty easy to buy one rather than building your own. If, afterward, you're satisfied that you know your precise requirements, and that the module you want to use is too costly, you can explore building your own.

BTW, for binary data, BPSK is at least as good as FSK, and it's easy to generate digitally. Manchester encoding, for example, is actually BPSK, and if you have two frequencies that you use to modulate your carrier, or if you use OOK (On-Off Keying) it is easily adapted, too. If you need to reduce the channel bandwidth, i.e. increase the data bandwidth, without significant modification, you can simply divide the Manchester encoded data by two, producing MFM. That's not conventional, but is pretty easy to process and quite effective.

BTW, If you use a PLL to receive/demodulate your FSK data, the error voltage that's subsequently integrated to steer the VCO is the modulating signal from the transmitter. Consequently, if you use the transmit-data to modulate your VCO at the transmitter, you should have the same effect at the VCO steering voltage integrator input on your receiver. For simple short-range, low power systems, it's pretty easy to cook up a simple modulator. Forward error correction and other sophisticated error management methods aren't necessary for really simple short-range systems. You do need a PLL with a lock-detector, though.

RE