Use data when possible, but when data isn't available, use common sense/logic and examples to make your case. You can also use empirical examples in similar market and describe how changes in underlying elements of that market create your market.
As for how to cite, formatting/type isn't a big deal - you may use footnotes, endnotes, APA, etc. Imo footnotes are easiest to follow - think Wikipedia citations.
Definitely use statistics if you have them and if they are directly relevant. If you don't have them, you probably haven't really evaluated your opportunity well. Some common ones to include are:
The more relevant, specific, detailed data you include, the better as it validates your idea and shows investors you know what you are talking about and you are not just full of hot air!
It also gives you confidence that your idea does actually have legs. After all, you don't want to spend a lot of time on a startup that is in a niche that is too small to provide you a return (for example).