I replaced 2 of them in my '05 Expedition a few years ago. Bought them off of summitracing.com, cheapest ones I could find that matched the dimension of the old ones. I cut the old ones out and welded the new ones in myself, but if you don't weld yourself, I'm sure an exhaust guy or a local welder could to it relatively cheap. My wife put about 75K on the truck before the neighbor kid wrecked it. Passed inspection 2 or 3 times in that time, too. Done the same to my wife's Tahoe (4 cats), and my '96 Bronco (gutted 1st cat, running aftermarket 2nd cat). I live in Colorado and these vehicles get emissions inspected on a dynomometer with an exhaust sniffer every couple of years. Mine have all passed with flying colors, and have been for years.
In my years of wrenching I've gutted many catalytic converters for one reason or another -- track vehicles, offroad trucks, clogged, ect... Most of the states I've lived in didn't require them. That said, there's virtually no difference on the inside (or the outside for that matter) between factory cats, and the cheap ones. They all look the same on the inside--1 or 2 bricks of extruded catalytic material, high-temperature wadding and pads to keep the bricks in place. If someone were to hand me some OEM, OEM replacement, and a cheap after market catalytic converter, I would not be able to tell the difference between them physically by what's inside of them.
There is a difference, though, and it's not what you think. For C.A.R.B. compliant cats, and this goes for OEM replacement cats as well, there is a certification process that is associated with the part number. This certification process is designed to validate to the EPA that the cats can do their job, and to quantify the results. This costs the companies that produce them quite a bit of cash, so these are generally more expensive.
If the rest of your exhaust pipe is in good shape you can save yourself a lot of cash by going with the cheapies. I will say this, though: look into WHY the catalytic converters need replacement. If its simply age and use, that's one thing (the ones on my Bronco were ~25 years old at the time). But if there is a problem--engine running lean or rich-- it'll need to be addressed or you're going to see issues with the new cats as well.
Edit: To address your question directly, there is no way for the engine computer to monitor which catalytic converter is installed in your vehicle--there is no means for the computer to read the model number of your cat. A CEL cannot be triggered simply by installing a different catalytic converter. The way the computer knows if the cats are doing their job is to monitor the oxygen sensors and comparing the output. This is why we have 2 sets -- generally 2 sensors in the head-end of the stream, and 1 or 2 sensors (depending of the vehicle) in the down-stream exhaust flow. The first two sensors are used for determining and reporting combustion efficiency. Fuel mapping is associated with this output. The second set of O2 sensors are used to compare the readings before and after the catalytic converters have done their job, but the output does not affect fuel mapping. If the new catalytic converters installed are even marginally functional, the computer will not know the difference. You're fine installing the MagnaFlow exhaust.
