Hi
In my opinion 10Hz-100Hz is not a good choice. OPAmps have still high 1/f noise at that frequency. Better 500Hz-1000Hz. If you choose 10Hz the low pass filter needs to be very sharp to avoid the presence of modulation frequency. You did not specify if you want a full digital lock-in or not. If you want a digital solution better use a dsp or ARM as the processor. I would use a mix solution. Check the "Tayloe Mixer" for the mixer.
Regards