Do you NEED a coach? No plenty of people complete double and longer without them. Is a coach a good idea? Quite possibly. I have been building up to longer and longer events over the past few years and my training has become more focused along the way. However my training is still not all that structured. I do not have much of a structured plan at all other than ride regularly. I know I am not getting the most out of my time as a result.
I am planning on working with a coach this year in preparation for the Everest Challenge. I interviewed a few different people in my area (SoCal) and have settled on a someone, though I have not started my training yet. One person has years of experience, is a really nice guy, and live in the same city as I do. Unfortunately he is just way too expensive for me, $200/month with unlimited support or something like $75/hr for ad hoc services.
The person I am going with will created structured 6 week training block tailored to my goals and availability. In my case he will be creating two 6 week blocks that build on each other leading up to my main event. He provides this service in essentially two level: 1) month to month program where we discuss my training on a weekly basis and make adjustments accordingly for $60/month - 2) $100/training block with limited review of progress but the plans themselves are tailored to my needs. All progress is tracked using Training Peaks
I am likely going to go with option two just because it is such a good value. I'm sure if I find I need/want a higher level of support I can switch to the month to month service. This person is not local to me (I'm in Pasadena and he is in San Diego), but another person who lives near me uses this guy and is very happy with him.
Another option if you are diligent and can figure things out for yourself would be to purchase a training plan from Training Peaks for $100. There are several plans on there. Problem for me is I don't have 5 days a week to ride and the person I am going to work with costs the same and will develop a plan around my availability.
Good luck with your training and your goal of completing a double century.