Thank you for your help.
The transmitted signal is centered at 40kHz +-500Hz.
I was thinking about Vcc=5V, Rin=27kOhm, C=1nF, Fin~40kHz, it gives exactly the same amplitude; I'm not sure that's the goal though.
I'll probably use a subtractor circuit along the lines of the attached image.
Also, off topic, would you recommend me to use and AGC or just amplify, filter, and then the subtractor and indicate the distance by a needle and tick marks (basically like an analog voltmeter, but I'm open to suggestions here too)? (also, given that I don't have much time to complete it)?