In reading your question you do bring up an interesting thought and one that does need to be asked,... "Will the officiating member of the marriage allow for flash during the ceremony?" Sometimes this needs to be addressed with the Bride and Groom before the event so that the paying party can step in and secure the requirements of the photographer.
I have always felt that the people paying for services should have a say in what services are provided. If the Minister, Priest, Rabbi or other official says 'no flash' then perhaps relocation is in order. It's a once in a lifetime event (hopefully) and capturing it is a once in an event effort. Posing after the ceremony does not capture the emotion of the event itself.
You say you can rent a D300,... then why not a D700 if that's more in line with what you will need.
Before the wedding actually takes place, do some test shooting in the church to determine the quality of light and how your camera (or rental camera) performs under the conditions of the day. Take into consideration that outside lighting may not be optimal so you may have to rely on artificial light. If you don't know how to do white balance here, you are going to have a post production nightmare in editing hundreds upon hundreds of images. Now is the time to do color management,... not the day of the wedding.
I'm not familiar with you so please don't take it personal that I suggest things that maybe you already have in your repertoire of skills. This is, by your own admission, your first wedding so it's not unreasonable for me to assume you have a certain gut-wrenching feeling about failure. I suppose we all have to start somewhere and this is your debut. My only hope is that you have the ability to think on the fly and to overcome immediate obstacles by adapting to your changes in lighting on the fly.
Your Nikon is a capable camera,... the question is, are you a capable Photographer?
Suggestion. If you can afford a stroboframe and external flash with a dummy battery (used in conjunction with external power), two battery packs (you will go through one reasonably fast and do need quick recovery time between shots).
It would be very desirable to have two camera setups with lenses already attached (fast lenses) so you aren't constantly changing lenses between moments. That's typically what a good event photographer will carry... it's called backup too. What would you do if one camera failed?... use the backup and go to switching lenses as necessary to complete the assignment.
Getting nervous?
You should be... failure is not an option here and someone is counting on you being able to do a good job. I don't know what you quoted as your fee but it should be enough improve your inventory of equipment and the references and referrals from your images may determine your future in photography.
Bow out... while you still have friends if you are not prepared to do this (you sound like you are far from prepared).
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