I will only comment on the questions I have direct experience with:
Where to buy? - I suggest used, if possible. Save a couple dollars, and help a member (money wise) with THE next part of their build.
How long do the fuel trims take to settle in? - The first 10 seconds, the car ran like poop. Then idled fine after that. Initially, the trims were all over the place, but I didn't have any driveability issues. It ran fine after the first start up. I've burned through about a half a tank so far, and it look like my trims have settled in where they're going to stay.
Cold start issues? - Haha! Not really an issue here!
Ideally, you would tune for the conversion, but this is a cheap hack ("hack" in a good way) way to run the corn. I will tune eventually,