It's very unlikely a customer will be excited about some white paper or document regarding your product. Implementing an idea is hard, as you point out in your post. Lots of people fail at this step. I think you just want to identify the problems that your application is trying to solve, and communicate that to the user. Generally, videos, screen shots, screen casts are the best way to reach the most number of people. Documentation generally isn't catchy enough for most people to notice.
I think so. You need to actually create a pretty functional version of your product. The best place to start is to list what problem(s) your product wants to solve. Then build the minimum amount of software you need to solve those problems. The end result has to be enough to be compelling for the user to use right away.
Yes. In my experience building a product first is pretty much the only way. It is really hard for a garage venture to get people excited about something without actually having them see the product in use. It can be done if you are well known in the industry, and have a great track record. But, this is pretty rare.
The main thing is to have a product that your potential customers can use right away. That's how you build creditability, and word out mouth following, which if you are just a one man operation is your main marketing too. You can worry about the other stuff later.
To list some of the other stuff you mention.
Documentation: This is not very necessary at this stage. I haven't encountered too many customers for whom this is a major stumbling block to buying your product.
Security System: It depends what you mean here. If you are worried about people stealing your software. I wouldn't worry too much about it in the beginning. The revenue you are likely to lose because of this is not going to be huge in the beginning. If you are talking about user security, then that is important, especially if you are building some banking or commercial software.
Licensing model: This is also not too important to get right in the beginning, because most likely the model you start out with will have to be tweaked over time. The reality is that there is no way to really gauge what your demand curve will look like. You have to experiment. Pick any model that sounds reasonable, and then adjust it over time.