I've worked with different types of emissions equiptment (mostly for diesels) and there's really no such thing as "reasonably affordable" emissions equiptment. In most modern engine labs the emissions equiptment is the most expensice equiptment--sometimes many more times as expensive as the engine.
That said, here's what you're up against. There a 6 (commmon) types of emissions that are measured: O2, CO2, CO, NOx, unburned hydrocarbons (HC or UHC), and particulate matter (PM).
O2 is probably the easiest to measure. A typical wideband O2 sensor can do a pretty decent job. Cost is probably around $50-$100, not including the module to read/control it. O2 is basically just a measure of your air-fuel ratio, so you're question about NOx being correlated to O2 is only true so much as NOx is correlated to air-fuel ratio. There is some correlation, but factors such as combustion timing will generally have a stronder effect than A:F.
CO2 is actually fairly easy to calcualte from basic chemistry. Fuel is your only significant source of carbon into the engine and CO2 is the only significant form of carbon leaving the engine. It's basically carbon in = carbon out. In fact, when CO2 is measured in engine labs it's typically most used for ensuring the fuel and air flow measurements are corrent anf they're are no leaks. They only reason O2 & CO2 are correlated is that as the A:F increases, there is more O2 and the exhaust is "diluted" so the concentration of CO2 goes down, but the actual flow remains the same (as long as the fuel flow is the same).
Almost always, at any given speed and load, efficiency & NOx will trend in the same direction, i.e. higher efficiency=higher NOx. Without getting into too much detail, NOx is exponentially dependent upon temperature. Higher temp = Higher NOx. Thermodynamics will tell you that higher temp also causes higher efficiency. My experience with NOx emissions is pretty much all with diesels, but I think what I'm saying about NOx hold true for gas engines as well (since you mentioned A:F < 18 I assume you're interested in gas). As far as measuring NOx, I think there are a few modern diesels with SCR systems that have NOx sensors. They're pretty similar to your typical O2 sensor. I think they're accurate to within ~15% to 20%. If you wan't more accurate than that, the lab grade analyzers I've worked with cost around $20,000.
With modern engines (gas or diesel) your CO & HC will be fairly low and generally trending opposite of efficiency. Typically they'll primarily be an issue during cold start, idling, and very light loads. I'm not aware of any production sensors, and the lab grage analyzers I've worked with are around $20,000. FYI, one analyzer can give you both CO & CO2, but you need a different analyzer to measure HC.
PM usually doesn't need to be measured on gas engines, only diesels. Generally smoke costs you efficiency. A smoke meter, which gives you an idea of PM (but not an actuall measurement) cost probably around $30,000. To get a actual mass measurement of PM, I would say you're looking at least $50,000.
So...if you've got plenty of money to burn on exhaust equiptment go ahead.
