I don't think you'd need to add any bracing/brackets, unless you are using them as an assembly aid.
Even just using but joints, it should be plenty strong enough considering the length of the joint.
Building a stand like that is basically just a box made out of plywood, it's relatively simple, and pretty strong. If you use 3/4", your stand is going to be built like a bunker (nothing wrong with that
). I think 1/2" would be plenty thick enough for that size stand/tank, but it might be easier to use 3/4", as it would give you a little more lee way when putting the screws in.
I'd also suggest looking at something like cabinet grade plywood, it's usually smoother, straighter, and will be easier to work with and less effort to get a nice finish.
If you plan on doing any other wood work/stand building in the future, I'd suggest looking into a pocket-hole jig. I had used one from a local tool-lending library (great idea, more cities should have them), and found it incredibly useful, especially for keeping the screws out of sight. I ended up buying the ~40 kreg version, and have been pretty happy with it.
Otherwise, you can just countersink the screws, add plugs, and sand them down, and it shouldn't be visible after the painting.
As to the finish, I also think just paint will be fine, and don't see much point to sealing the inside of the stand. Just sand the stand pretty good before painting, especially the joints/plugs, and some filler or something to plug any gaps/cracks/voids, use a primer, and follow the instructions on the paint can.