You shouldn't be buying stuff or spending money on an unpaid gig, IMNSHO. You didn't need that new gear to sell them on your current body of work, and depending on timeline you may or may not have time to get familiar with it before the gig. You mentioned borrowing some nice gear from a friend. What happens if that borrowed gear takes a beer bath on the dance floor or tumble down some steps during the gig? This stuff happens at weddings. It could happen on your 12th wedding, or you could have two or three catastrophic incidents on your first. Weddings are full contact, high pressure, no holds barred, and very psychologically rewarding. I hope you've done the math on all that.
Unless I missed it: I'm surprised no one has mentioned a contract. You should have a contract for a few reasons:
1. A model release so you (or your subsequent business interests--say you incorporate down the road) can use the photos on your website. That may feel ostentacious now, but what if you get a kick ass image and then later you find out they're more private and get iffy about you using a prized image that they paid nothing for an you worked your butt off to make?
2. Clarification of expectations. You're not a professional, have no experience doing this, and don't have a full array of professional gear, such as cameras with dual card slots. Make that abundantly clear and have them initial every page before signing at the end.
If you wont see them before the gig, email them your finished contract and make sure they're clear on it. That will be there time to object before you present it to them at the day of the event for signature.
I'm sure there's other things but I'm too lazy to think of them now.
"The ability to destroy a planet is insignificant next to the power of The Force." -Darth Vader.
Therefore: don't obsess about your gear, and think about ways to do things simply and beautifully. For group photos you'll probably do best finding open, clean (undappled) shade with uncomplicated backgrounds. Adding light for groups >3 gets complicated and is usually better done with a strobe or two. A large window will look great for bride and groom getting-ready portraits.