My .02 hopefully to add the to good advice so far..
Its rare that the tank leaks at all, almost never is there a really big problem with the tank. If you dont see anything visually wrong (like a big crack or hole), I would not worry about the ballast tank before getting the boat to the water. Make sure you understand how the valve and the vent (and vent plug work) and once the tank is full, make sure the valve is closed tight and the vent plug is installed. Then check for water in the bilge fairly often the first time out.
I would mainly recommend to rig everything up in the driveway first before hitting the water on a day with no wind. These boats often come with a great mast raising system, figure out how to use it. Raise the sails, rig the sheets, you likely have some way to reef the main, try and figure that out. If the jib is hank on, can you drop the jib from the cabin. Important because you may be dropping the jib because of strong wind and having to go up on the foredeck adds risk.
If you are familiar with small outboards, you may already know this but they have water pump in the lower unit that must be in water for lubrication and burns up in less than a minute if the outboard is ran out of water. Several ways to make sure the water pump is lubricated such as running the outboard in a large water bucket. Make sure your outboard is running reliably before you go out, it ends up being an important safety item especially if you are new to sailing.
Look at the valve on the bottom side of the hull. On most D and S models, the valve is recessed a little so that even when the valve is open, you cant damage it when loading the boat on the trailer. But on some very early D models, the valve was not recessed. Probably not a big deal if the valve is flush with the bottom, I would just make sure the valve was closed (not sticking out past the bottom of the hull) when loading on the trailer.
The rudders on theses boats is long enough to drag on the ground if left down when you pull the boat up the ramp. You likely have some sort of method to raise the rudder from the boat.. raise the rudder once the boat is on the trailer and before you pull the boat out.
You also of course need to raise the dagger board to get the boat back on the trailer. But keep in mind that when the dagger board is raised, the boat does not steer as well. When going slow, the rudder may also not steer the boat very well but if you can vector the outboard thrust along with the rudder, the boat becomes very maneuverable.
Unless you have someone with you who knows the boat, first time go out in light winds.. Good luck, I think that is a great boat.
Some ideas also to review in a similar post from last year
https://forums.sailboatowners.com/index.php?threads/checking-in.186003/