A coach will help you with your physical training, but I think as far as helping you to understand racing, a team is better. I joined a team first (after having done 1 race unattached) and hired a coach my second year.

Now there are people on my team of all abilites who use coaching and I think that no matter where you are starting from a personalized plan will certainly help you reach your goals a bit faster, but for me it was the same reasoning as buying a new lighter bike. The expense would be worth it when I was close enough that the edge I had by using a coach would lead to results.