Having an RC hobbyist on the project really sounds essential. It sounds as if you're going to need somebody to physically build and connect this system so obviously it would need to be somebody local to you. Are you in contact with any local model clubs? I would bet there are people nearby with the skills you need and willing to help you. Over the internet it's hard to do more than offer advice and encouragement, and that won't take you far.
I understand that you have a 'sip/puff' interface that generates some DC output for the four possible inputs conditions. You don't mentioned the voltage/current characteristics of those outputs and you would need to know that to interface it to anything.
Have you figured out what sort of radio link you'd be using between you and the model? What sort of range/speed are you envisaging?
Have you figured out the details of how the sip/puff inputs would relate to speed and steering? I imagine the vehicle will need to keep moving while it steers so perhaps you want to be able to set the speed and have the model continue until you tell it to stop - but perhaps you have a different scheme in mind.