I dont think it is too difficult to do both amplitude and phase in the same measurements. Is it supposed to be all analog (i.e. outputs a voltage/current that represents the phase/impedance) or do you have some type of analog-to-digital conversion and then use a micro to handle the rest of the system?
Since you are setting the frequency yourself, if you are supposed to do the work in the digital domain, you can do a very simplistic DFT that would just need 1 multiplication per sample. You would then have both the amplitude and phase of the output signal. If you also capture the input signal, then the change in phase/amplitude is easily measured.
In the analog domain, you would build a phase detector circuit that would give you a voltage that is linear with respect to the phase difference between the input/output signal. The RMS amplitude difference in conjunction with the known source impedance could then be output.