Any snake is a good snake. It depends on your situation. Have you got the space and money to spend on a potentially 12-foot animal? If not, then coastal carpet pythons are not for you. If you want something slightly smaller, go with a bredli (centrallian carpet python) or a MD (inland carpet python). If you don't have much space at all, go with something from the Antaresia group (such as a spotted, childrens or stimsons python).
If you have all the space you want, then you could go with an olive or scrub python. You just need to know that these snakes get to massive lengths and can give nasty bites when they're adults, and that it's always a good idea to have two people when getting them out.
What you need to do is research, research, research. Find what snake(s) appeal to you, by colour, size, cost, whatever, then go with it. I personally started with a spotted python and can safely say that I wouldn't swap her for the world.
As for your other questions- a bitey snake depends upon the snake itself. Generally younger snakes bite a lot because they feel threatened. A bite from a juvenile doesn't hurt. Handling and time will usually calm the snake down and allow for a placid animal. Remember, to them you're a HUGE hot hand that's going to eat them. Once they get a bit bigger, they see that you're really not as big as they thought, and that you're really not going to hurt them. With size grows confidence.
As for the age, if you're trying to convince your wife, then I'd go with an established-feeding juvenile (usually two-three months or so). This way, your wife can grow with it and not be daunted by an adult snake. A younger snake is usually easier to handle, because their bites don't hurt, and it's easier to condition them to being used to handling. Just remember that snakes often stress, and this can lead to a huge amount of problems from going off food to death, so it's best to let them be for a while when you first get them, then slowly start to introduce yourself to the snake.
I must say, I've found snakes a lot easier to deal with than bearded dragons, and a whole lot easier to care for.
Hope that helps.