First of all, I wouldn't buy anything from Petco or any pet store without thoroughly inspecting the snake for (1) overall condition (muscle tone, etc.), (2) temperament (i.e. is it calm or does it freak out when handled?), (3) parasites (can you see mites or ticks - sometimes you can't, so be careful!), (4) health (i.e does the snake have any discharge from its nostrils? Are its eyes clear? Is it alert?), (5) environment (a snake kept in a dirty cage is probably not healthy and is a bad investment).
Second, I would not recommend riding around with the snake in your car. A new snake can be easily stressed if it isn't used to you or your habits. Improper environment (too high temps, etc.) can also stress the snake. Another thing to think about is most snakes are escape artists. If the animal gets loose in your car, you may have no way of knowing where it is, and this type of distraction could be very dangerous for other drivers. Driving around with your snake is just asking for trouble, and after the python/boa law issue, I don't think any snake owner wants to see a similar law enacted for kings.
The reason people are telling you to get an older snake is they are often much easier to deal with than hatchlings. These older animals, if in good condition, are usually feeding well and tend to be a little calmer than your typical nervous hatchling. In a nutshell, older animals are usually easier to deal with especially for a first-time owner.
The last thing: snakes are individuals. Just because one snake of a given species is nippy, it doesn't mean that all examples of that species are that way (the reverse is true as well). This is why it is so important to hold the snake in its normal environment where it is comfortable so that you can assess its temperament.
If you're looking to buy a first snake, you might try asking around the forum to see what everyone has in the incubator. A good reputable breeder should be willing to help you find the right snake and assist you with any problems you may have with that snake.
Sorry for such a long-winded post, but I hope it helps.
Orocosos