1. Is it necessary to be a niche forum, or could it be a completely general forum where people can speak about anything?
The larger it is, the harder it is to make it popular.
2. Can it be started/maintained/grown by a single person?
3. How can you be promoted?
I don't need to be promoted, thank you
; now, if you are talking about the forum, you can start with your friends, people with similar interests on a Google group, advertising, paying for posts. You could also try banner exchange programs like this one
, specially while you are starting.
4. How do you build the content in the first weeks and months?
5. Once started, how can you bring on visitors?
6. Once you brought visitors, how do you convice them to become members?
Give them something challenging enough to motivate them to give their opinion, or something useful enough (like free promotion, free comments or free help) to motivate them to join.
7. How do you find/pay moderators?
Usually Internet forums will do. You can pay either an agreed amount of money or perhaps including their Adsense or other advertising network on your site, so they can earn from there.
8. How many crew members needed for a 10k uniques/day forum?
That's too hard to tell.
9. Should you start it directly with a paid board (vBulletin) or could you start it up with a free one (PHPBB) and then migrate it?
If your target forum software has an import tool, you can go for the latter.
10. Is it possible to migrate everything from a free board to a paid board.
It depends on the forum software. Vanilla
11. When should you start monetizing it (showing ads)? From the beginning or after you have settled your community?
From the beginning is OK, as long as you don't abuse.
12. When do you need to rent a dedicated server? Can you start with shared hosting?
Yes, you can start with shared hosting.