Hey Nathan,
Welcome to APS! Wow 3 marine aquariums eh! You must show us some pics one day.
Snakes are awesome and can be very interactive. My first snake was a hatchling Wheatbelt Stimson's Python from Snake Ranch. It was so placid and calm, never bitten me, even to this day years later! You will certainly get lots of suggestions from different people here on the forum, but for me anyway, getting the Stimmies was a really good start. She was just so good natured, had no problems feeding and was so easy to care for. I also have BHP's, Children's pythons as well as Darwin Carpets. Probably wouldn't recommend Darwin Carpets as your first snake but any of the smaller Children's group - i.e. Children's, Spotted or Stimsons, IMO make a great first snake. My BHP's are also fantastic, love them! They're also very good natured....well, my ones are anyway.
It's your choice whether you want something smaller and easy to handle to begin with, or jump into something a bit bigger from day 1. There are always the calm ones and the crazy ones in nearly every species.
If you're buying a hatchling or anything young and small, it's best to keep them in a smaller enclosure first. Don't put them straight into a 4ft enclosure to begin with as they won't feel safe or secure. Snakes like to squeeze and hide in tight places when they're little. The smaller the space, the better for hatchlings. Yes, it sounds strange I know!
If you like Reptile One stuff, I am using a few of their black hatchling boxes at the moment, which is like a black acrylic box with a clear sliding lid and inside has a thing to hold the water container so it doesn't get knocked over. So far, works pretty well for me. Not cheap but works pretty good and I love the black acrylic look.
Or you can make a "click-clack" yourself. Search this site for "click-clack" and you'll find plenty of pics and instructions.
Hope that helps ya mate!
Cheers.