Freshie maybe going salt with my 300 gallon?

[kien]

This will be a great set up and I think you are already on the right track with the correct mindset. You already know that it's going to cost you time and money

As you suggested yourself, just start out slow. Rocks, salt, a few fish. That's really all you need beyond what you already have. Things like sand, skimmer, lights, is sort of a personal choice and usually ends up depending on what your ultimate goals are with the tank.