@cr0sh,
Thanks for those links! I have to admit that building something from scratch does feel good regardless of the material cost. 
Getting back to the one sensor question, The main challenge is to design a transmitter that is powerful and then at the same time to design a receiver that is sensitive.
In a one sensor design, the transmitter side usually is not bridged since the transducer needs to be multiplexed between sending and receiving. If the sender needs to be bridged the circuit will become even more complex.
Also a good analog switch chip that can deliver the power needed and has a low noise level is probably going to be expensive. But I think I might experiment with a single transducer design sometime just to see it myself.