You should definitely have a marketing program. You could make the next Skyrim . . . but if nobody knows about it then it doesn't do you any good. Likewise, you could make a really awful game, but if you have a large audience you're still almost guaranteed some downloads.
My two cents on ads: Avoid them. I feel like all they do is annoy players and detract from the look of your game. Besides, most people by now have seen so many ads that they automatically look for a way to get past them. Sure, you could add a feature where you have the player pay to get rid of the ads, but by doing that you're basically saying, "Pay me to get rid of something that shouldn't have been there in the first place."