I presume you are talking about a lambda sensor? I imagine you'd need a wide band sensor, of which Innovate LC1 is the best known. You can buy generic wide band sensors quite cheaply, instead of a 2nd hand one in unknown condition which anyway will all be narrow band. The expensive bit is the controller which the wide band needs. I've not heard of the diy kit you talk about but there is a site called 14point7.com where you can buy a diy controller kit. I've no experience with this, only researched it out of interest.
I would think you would need 2 sensors, one in each pipe, just before the Y.
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