I am in the middle of building a stand. Mine will be in the dining room so it will be a little fancier.
2x4's are fine. I also recommend the 3/4" plywood on the top. It will help with any constructions unevenness.
Sealing the wood is a good thing, just to be safe. Even if water does not get on it, the wood will be exposed to higher humidity, so sealing is better.