I'm trying to figure out what the alternator has to do with your IAC valve. The alternator basically supplies electricity for the car while it's running and keeps the battery charged. If the alternator is bad, the car won't run, nor start. Is that happening or is it just stalling out when you come to a traffic light or stop sign? The IAC valve def could be the reason it's stalling. I'd ask specifically what the alternator has to do with your problems.
Also, $1600 to replace the alternator and IAC??? That's WAY too much. An alternator costs like $100 and the IAC is ~$200. The alternator takes all of 15 minutes to remove. It's literally one long bolt that secures it and allows it to pivot up and down (to tighten the accessory drive belts), and a couple of bolts used to adjust the tension. Someone with zero experience could remove and replace it in an hour. There are YouTube tutorials for this. My ex had a 99 Camry that had a bad alternator, so I'm familiar with the process. So for that alone, it should cost no more than $300-350, depending on their hourly labor rates.
As for the IAC, a lot of times you can clean it and that will fix the problem. Not sure if you're up for that, but there are YouTube vids to instruct. If it IS bad and needs to be replaced, that takes about 30 minutes to remove and replace. So an hour of labor + the part cost. If both need to be replaced, your bill should be no more than $800. If you, or a friend, is somewhat mechanically inclined, and has basic tools, it's a very straight forward process and will save you $300-400 in labor.