The bandwidth is probably the biggest problem. I'm not sure how much traffic a FAQ site would generate, but if you keep it clean and use mostly text only I'm sure it could be run on a DSL or Cable modem connection, or at most a free host somewhere.
As for design vs code you are completely correct. While I personally prefer to sit down and start coding something, it's alwasy best to design it to be what you want before you start coding that way you won't run into any nasty surprises.
So what are we talking about:
1 - database driven question answer site (kinda like a forum but more structured).
2 - needs the ability to have questions asked and answered.
3 - needs the ability to show PHP code in a readable fashion.
4 - needs a robust search engine.
5 - needs well thought out catagories (can a question reside in more than one catagory, I believe so).
6 - should have the ability to email the person who asked the question when there question has been answered.
7 - should questions be allowed to be addressed to a specific person, or just go into a pool to be answered?
8 - can anyone ask/answer a question or is it limited to only registered users?
I'm sure there is alot more, but that's off the top of my head.