My circuits was intended for higher freq., 500 kHz or so. Idea was to tune L and C (photodiode) making it selective to specific band. It won't work on 38 kHz, you need 100x bigger inductor -);.
So, if you will use a laser directly pointed to receiver, I don't think you need amplifier, instructables project is correct, I think.
My understanding of MCP3304 , that you are setting conversion rate by applying desired freq.to CLK pin, according to picture 6.2 you need 100 x 21 = 2.1 MHz clock to get 100 ksps.
