It all depends on your goal: if you just want to have quickly a camera streaming setup, this would be the easiest solution.
But if you actually want to learn how to do it the hard way, then buying the original and making it yourself is far more rewarding.
As for where to find information, the Wiki is generally a good place to start, then you have to skim the forums, where people already discussed how to set up a camera for streaming.
Regarding the custom web-pages, the entry point much depends on your previous knowledge: if you already know about web technique (HTML, Javascript), then it is pretty straightforward for simple pages.
For more sophisticated pages involving partial refresh and interactivity and if you know nothing about it yet, try learning about Ajax and study the contents of HTTP headers.
Now, if you want to provide router-side dynamically generated contents, you will have to learn CGI and the language used to generate the dynamic pages on the router:.
If you are using LuCI, then this is Lua, and a good start point is on the corresponding http://www.lua.org web site, where you will find a lot of tutorials.
For LuCI itself, it will be interesting to look at the materials provided at http://luci.subsignal.org/trac, and to study the existing LuCI code itself.