Running Lean

We recently bought a 2001 Saleen S281SC and it has these error codes:

P0136 Downstream Heated Oxygen Sensor circuit fault - Bank No. 1.
P0156 Downstream Heated Oxygen Sensor circuit fault - Bank No. 2.
P1406 Differential Pressure Feedback Electronic Sensor circuit Downstream Hose.
P1131 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 1.
P1151 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 2.
P0171 System Too Lean - Bank No. 1.
P0174 System Too Lean - Bank No. 2.

The seller claims that the gasket he ordered should fix it and that it will be an easy install. Our mechanic tells a different story. Also, since we are in the state of CA, the seller was supposed to be responsible for smog certification, but because of the CEL, we cannot even do that.

Our mechanic said we would need to have several gaskets around the supercharger replaced, at the throttle body, intake manifold. It's a mess. On top of that, we need to have the oxygen sensors replaced and find out why coolant is leaking. Do you guys have any advice? Any help is greatly appreciated. Thanks in advance.

EDIT: Also, the idle rises and falls over and over when it is stopped. Is that the IAC? That's really strange to me, the car only has 9000 miles and it's carfax and autocheck are clean.

As far as I know, the car is stock Saleen. So i'm guessing no MIL elim. How do I check if it is sensor 1 or 2, and if they are plugged in?

(as you can tell, I am very new to this)

First of all, thanks for the advice. Stangnet absolutely rocks. Within minutes of posting my question i get a response.

As for the mils, I'll order one. Is it a difficult install?

On another note, I had a different mechanic look at it today, and he cleared the error codes. So far, they haven't come back on. When they do come back on, he can see which codes came back up and go from there. After we get the gasket, he wants to do a smoke test to see where the leaks are. Will that detect vacuum leaks as well?