I think the biggest concern is neck strength. There is potential to get jostled. I have seen recommendations for a year old. You could probably do earlier but I personally would not do earlier than 10 months. Expats method is probably the only way to get them out earlier.
If you plan on having more kids, get a trailer sooner rather than later.
I have never used a rack carrier either and I don't plan on ever using one. If I were to get one, it would probably be the Kettler. I think Rhode Gear makes one that looks decent too.