Before I started doing weddings I was a second shooter for a long time. Before I was a second shooter I would say I was relatively strong in my skills, and I was able to produce consistent results in most situations. Now a few years later I am still challenged with each wedding that I shoot. Experience is the best advice I can give you, and at this point you do not have it. If I were you I would hold off on shooting this one and take the time to gain the experience you need and then maybe consider weddings in the future.
I understand where you are coming from as I have read a lot of wedding posts on here. I love your work by the way... I am not going to claim to be a wedding photographer by any means.. I am just doing this for a friend and I know I can do a better job than the people she was going to hire.
She really would like to not spend 800$ on a photographer with unimpressive work. I already told her I would do it, and I still have a while to prepare.