General Advice
Being a wedding photographer means that you have to stay one or two steps ahead of the bride and groom, know your equipment like it is an extension of your body, and be able to create beautiful images that tell a story. You already have a great understanding of your equipment(sans OCF) and are already a successful portrait photographer. What will be more difficult for you is to be completely on top of each unique situation that a wedding day can bring, such as extremely dark lit churches and reception halls, subjects that are drinking too much, the demands that a full 8 hour or more shoot can put on you and your equipment, etc. It also may be a challenge for you to successfully put together an end to end story of a wedding day. This is something that can be taught and learned, but just keep in mind that normal portrait sessions don't quite tell this kind of story(although they can somewhat).
Natural light photographer is a marketing term that to me means I don't know how to properly use a flash. You were gracious enough in your question to note that this is something you aren't an expert at. Most photogrpahers will note this on their websites or marketing materials as some sort of a selling point. A good photographer will use natural light where appropriate and additional on an off camera lighting as necessary, in a seamless and beautiful way. Being able to successfully utilize and manipulate all kinds of light both artificial and natural is what being a great photographer is all about. It is absolutely essential to your success as a wedding photographer that you learn off camera flash. With the wedding a year away, you certainly can get your feet wet and become much more versed in the skill. It is challenging for most photographers and even a year of intensive self teaching may only be the begining for you. On the other hand you may learn quickly and be successful in a much shorter time. At any rate, being competent in off camera flash is a necessity for wedding photography. You will find yourself in locations that are simply too dark, even with the great high ISO performing cameras of today paired with wide prime lenses. But it isn't just dark locations that prove OCF as useful, as there are many situations where you will benefit from adding flash to even out the light, provide dramatic effect, or suppliment what exists for the shot you desire.
You noted one interesting point, in that you currently already focus on engagement sessions. I specifically advise wedding couples to book the engagement session with the same photographer that you plan to have at your wedding day. It is a great way to "trial run" how the couple is in front of the camera, learn a bit about each others personalities, and also for the wedding couple to find out if the photographer is a dead-beat or not. Unfortunately right now you might be missing out on great engagement photography opportunities since you are only doing that and limiting yourself to not currently doing the wedding part.
Finally, if I can give you one piece of advice that above all else I believe is important, I would suggest becoming a second shooter at a wedding first before attempting one on your own. You may have to start out as more of an assistant, carrying equipment and gathering people for necessary shots. But after proving yourself at that task, usually a photographer will be able to utilize someone they trust as a second shooter that can actually take photos at the event. Second shooter experience will be invaluable to learning the ropes of timing, flow, and expectations on a wedding day. You certainly don't learn everything from this role, but you learn a great deal that will benefit you when/if you desire to be the main shooter. See this question for more on this topic: How do I go about becoming a second shooter for a wedding photographer?
Overall, my recommendation to you is to determine where you see your business in 1, 2, 5, and even 10 years. If you want to grow, without burning out, I would suggest that weddings may be one of only a few options. They can be very lucrative when compared to what may require hundreds of portrait sessions a year. If you do decide that you want to jump into wedding photography as your business grows, I strongly encourage you to first second shoot a wedding(or multiple) before diving into an event by yourself. Beyond that, a very strong understanding of OCF is necessary and something even your portrait business will benefit from a great deal - so I would advise getting started on that regardless.
Your questions
Should you let the client know of your inexperience?
Absolutely. To not share this fact is a misrepresentation. It is very likely that they find your portrait photography work great and also are aware that your portfolio does not have wedding images. But being upfront and honest is something that clients will appreciate, and likely lead to some formal(legal) agreements based on any possible issues that you anticipate. To do this properly, don't just add a clause to your contract - first share with them your experience in portrait photography and explain that while portrait photography is a major component of a wedding, it also includes situations that you are not yet well versed in(but willing to learn as much as possible before hand).
Is it a horrible idea to agree to shooting this wedding?
Based on the fact that you already are successful in the portrait photography area, I would say that it isn't a bad idea at all. Most portrait photographers find that weddings can be highly lucrative, and assuming that you enjoy the fast paced all day nature of the event, and don't mind the typical weekend commitment - you may find that weddings are where you strive to take your business in the future.
Should I even attempt this?
That is a question only you can answer. Do you want to grow your business in this direction or not? Does the idea of photographing a wedding excite you or are you only considering this because someone asked for the service? Only you can decide what is right for you, your business, and this potential client.
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