first of all, your "donation" to their wedding will be greatly appreciated. It's a lovely gift.
Secondly, you might want to consider renting the equipment you need just for this event rather than purchasing. There are several good rental companies and they have everything you could possibly need -- including. . .
An extra body: you may want to minimize the number of times you change lenses and this is the best way I know.
The 17-55 F2.8 IS is a great lens and it will do a lot for you. You might want to rent the 10-22 wide-angle zoom just for those total room shots. Also, if you plan to shoot available light for most of this, you might also consider renting the 70-200 F2.8 IS. It's a great wedding lens.
The 580 flash unit will be great for portraits or fill light. Especially for the reception.
I'd advise you read up on the basic approaches to wedding photography (traditional/formal and journalistic) and ask the bride & groom which style they prefer. Also, figure out which style you prefer. If everyone's in agreement, then your work should meet their expectations which is always nice.
Also, figure out how you'll deliver the images -- prints, a book, digital picture frame, album, etc. Again, managing expectations is the hardest part of wedding photography in my opinion. Which is why I am loath to do it
So, in brief, if you want to purchase some equipment and this gives you a good excuse then fine but if you'd really rather not purchase, renting is a very good option. If you and the bride & groom are on the same page about what type of images you'll shoot and how you'll deliver them, then they won't be disappointed. And that's the most important thing. I have no doubt you'll do an outstanding job.
PS if in doubt, let me know where the wedding will be and I'll try to help out.