Are you being paid? I'd forget about the lens unless you have disposable income to play with. It is a very nice lens but you currently have decent coverage. The 85 will be a sharper and faster than your 28-75 but how much difference will it make based on your friends expectations. Your 50 1.4 is fast enough to cover low light situations.
Where is everything happening? Standard Wedding - church, formals at some other location, reception at some hall?
I know there are natural light photographers which is great if it is available. You will probably wind up in a dark venue and unless your camera can shoot very high ISO you will need a flash. Even with with the 50 1.4 you will need some fill.
Now the flash. This opens a whole new world. One week is not a lot of time to prepare to really understand how the flash and camera work together. What I mean by that is how to control them separately. Even though the flash is attached to your camera they have two separate jobs. In fact your camera does not care about what your flash is doing and the flash does not care what your camera is doing.
Your camera exposes for ambient or available light using the cameras light meter. When you put your flash on the camera the light meter has nothing to do with flash. It can't because the flash has not fired yet and your system cannot predict flash output.
So as I said your camera has a job and the flash has a job. The camera exposes for ambient and the flash exposes for your subject/s. It is often referred to - two exposures in one.
Watch this video. It is a Pocket Wizard ad but has great animations. At minute 2:30 you will see the shutter open and expose the ambient light or first exposure. It may not be much of an exposure but it is an exposure. Then the flash fires to illuminate the subject - the second part of the exposure. Note: You don't need PW because that is for off camera high speed sync. Also you don't need to know anything about HSS now but it is interesting to see how your shutter and flash work together.http://www.pocketwizard.com/inspirations/tutorials/pocketwizard_controltl_optimiz/
I'm not sure what your camera is but if you get a flash I would suggest you keep things simple and put the camera P mode and set your flash to ETTL. ETTL fires a pre flash to determine correct exposure of your subject. It is a good tool and gets you close but is not an exact science. The next step is learn how to adjust the Flash Exposure Compensation or FEC. After each shot check the histogram, adjust the FEC as required and shoot again. My guess is there will be enough white to work with. That is all you need.
Here is a link on how to interpret the histogram. If you get whites right the rest of the exposure falls into place. Again remember this is for your subject, not the ambient or surrounding light. Scroll down to the gent holding the white towel. http://super.nova.org/DPR/Histogram/
So when using the flash we have the camera on P mode and flash on ETTL and we adjust FEC as required for subject exposure. Now the best way to use flash is put the camera on manual when using flash. In P mode you don't have a lot of control over depth of field. Actually there is little difference between P and M modes. In P the camera controls shutter and DOF and in M you do. This gets a little complex and I can get into that if you wish but like I said one week is not very long for this learning curve.
Next when using flash I would suggest shooting with the highest ISO you are comfortable with. 1600 - 3200 even better. This brings in more ambient light so the flash does not have to work as hard, there is better balance between flash and ambient thus the images look more natural, not like your subject/s have been nuked.
So far. Camera on P, flash on ETTL, learn how to use FCE and use a higher ISO.
If you are going to use a flash I would get it yesterday and start practising. If you do later I will explain why the flash head rotates (bouncing the light) and how that gets better, more even/pleasing flash exposures.
Here is a list that may help you.http://digital-photography-school.com/wedding-photography-21-tips-for-for-amateur-wedding-photographers