1) As many as you like (non-vBulletin limitations aside).
2) Yes, most of the time, the manual will have the answers. However, if you need to code your own modification, or create your own style, then the appropriate expertise is required (of course).
3) Depends on how you want to generate revenue. Advertisements can be placed practically anywhere.
4) No. Many people start their forums on shared hosting, and move to more powerful hosting in the future.
5) I do not know of such a modification available.
Please do let us know if you have any other questions.