same as Phil here: very nice project and thank you for the link.
maybe you should look into handling interrupts for the sense wire. i imagine that the inducted (?) signal will not only be weak but also quite short.
as for the opamps, i can't help you very much neither. maybe wait for your ferrite cores to get an idea of the signal you'll be getting. if you could use an analog pin to sense the signal, that would be by far the most simplest solution.
keep us informed, please 
kuk