Placing two oxygen sensors in the exhaust before the turbo is a mistake. We might need to do a little more work at figuring out which piston the current emission pattern belongs to and we'll probably need a heated sensor since the exhaust after the turbo will be significantly cooler, but we should avoid obstructing the exhaust pulse to maximize its effect on driving the turbine.
The good news is that we'll only need one probe, the bad news is that heated lambda probes are considerably more expensive. Rockauto has a universal heated oxygen sensor for $20, but charges $32 for shipping. Ordering 10 units brings the price down to $24/piece incl. S&H. Amazon had one for $40 + S&H.
Note on tablet PCs:
Since we'll be wearing gloves we'll need a device with resistive not capacitive screen, which means we're in luck: the technology is a little cheaper!
For an extra $30-$50 we can get VERY powerful 7" tablets with ARM cortex-A9 processors and 512Mb RAM. Two cores @>1.0 GHz should be sufficient to run all the stuff we might want to add later, such as a rear view camera with nighttime vision enhancement post-processing and anti-locking brakes (which are both on my wish-to-do list). Supposedly, most android apps are single-threaded, so having an extra core to deal with time-critical operations can't hurt.
Although a 7" (16:9) device fits well (the screen has roughly the same hight as the speedometer), so depending on the top and bottom bezel-width we might be able to replace our dash-board with an 8" (4:3) device. We'd get an extra 120 pixels in height...