I would argue the advantages of joining a clan are exactly the ones you state in your post. You gain contacts with more experienced and more knowledgeable people who have an incentive to help you develop as a player.
However the value of this, at least from my perspective as someone who is clanless, is entirely dependent on the clan you join. If you take the time to find a clan that has members who have a similar philosophy to you, I'm sure it will be a positive experience.
Whether or not you actually need to join a clan or not to gain all these advantages is debatable. If you contact a more serious clan about joining they will often involve things such as test games where your ability is evaluated. The process of doing that will almost certainly improve you as a player and help increase the number of players you know who may be willing to help you.
In answer to your specific questions: -
- Requirements of a clan and your experience of it are entirely dependent upon the clan you join.
- Clans do have their own private message board.
- The time commitment is probably down to how much you want to put into and get out of the clan. The value to you for doing this though is down to how closely your values align with those of the clans.
In your position I would try and get to know this person who is offering help to you and suggesting their clan to you. I would probably have a look in the forums to see the behaviour of members of this clan. I'd take any advice and feedback on how I play, and probably be asking the same questions you pose in this thread directly to the person who may be giving you the opportunity to join the clan.
I would list the sort of clans I think you could be interested based on your post, but I have never taken the time to find out about clans. I can tell you based on my experiences of playing on the ladders however, that I do know the clan ONE! has some good people in it who could help you improve as a player.