Whatever you choose to buy, I am sure you will enjoy. Baby snakes are nice, because you have the opportunity to watch them grow and you can measure the progress that you and your snake make. They tend to eat a little more often, but that's ok.
Sub-Adults are nice, because they are just about guaranteed feeders. Some times they can be a bit wormy, especially if they haven't been handled.
Adults, if you are not used to them, can be a bit intimidating. I wouldn't really suggest starting here, because you miss the growing pains. Younger ones, give you the opportunity to learn about your snake and possibly mold it, to your liking. Older snakes are very set in their ways and if they haven't been handled, a lot, you will probably, at this point, not be able to get them used to it.
When you go to buy, just make sure the snake is healthy, has been eating consistently, hasn't had any major shedding issues, isn't aggressive and is already feeding on Frozen/Thawed rather then Live.
When you hold it, check for good muscle tone and a firm body. Make sure the snake is moving good, doesn't have any spinal kinks, doesn't have any wounds and is scenting the air. Check the snake for signs of parasites by looking near the eyes or along the belly. You can also look in the water dish, for something that looks like sand (sign of mites).
Look at the viv that the snake is kept in and see the condition. Hopefully it's clean, there isn't a build up of feces or urates, there is fresh water and is free of foul odors. Look at the thermometers and see what temps the snake is kept at.
The last bit of advice I could give, is make sure it comes with some sort of health guarantee. Moving a snake is very stressful to them and you want to make sure you are covered. Get any numbers or emails that you can, so if you have a problem you can give a call. Make sure all of this is in writing, too!
This is all assuming that you can handle your potential purchase and your not ordering it, on line. If you are going to order a snake online, make sure the breeder is reputable by checking the BOI
http://www.faunaclassifieds.com/forums/forumdisplay.php?f=13
Good Luck,
Wayne