+1 on point #4. The one big glitch to Zone 2 has been the limitation that you can only output analog signals to Zone 2. You have to download the manual and read it...sorry, no easy answer. As of this year, digital sources to Zone 2 is now becoming more common, showing up in Denon products for example, probably others.
If you are focused on a model a few years old without digital source to Zone 2, there is a work-around: use an analog iPod dock. You might give up remote control, but at least audio would work.
The other work-arount, which actually is pretty good, is to use an AirPort Express, and it's analog output. You'll have full iTunes library streaming, and control with the Remote app on your iPod Touch or iPhone.
Current Apple TV has no analog out.
AirPort and Apple TV are close to the same price. But, if you're getting an older cheaper AVR to save money, then spending $100 on one of those devices, you might consider paying up a little a current model that has built-in AirPlay, and will do it to Zone 2.
As to answering "What's the cheapest..." you have to figure that one out by reading manuals to find the unit that works for you, then checking prices. It's not something most people would know unless they've done that research project themselves.
I tend to not recommend the cheapest of anything, though...it usually has some hidden limitation not discovered right away.
Look at Pioneer for built-in AirPlay capability in lower cost units, though I always recommend Denon first. Last I looked at entry level Onkyos, the didn't seem to hit all the required features.
Always check for built-in WiFi too, unless you can wire to your net. It's often listed as a "feature" when it's actually an add-on. Wire to your net if you can, it's better anyway. Always research the Zone 2 capabilities.