Your kit lens really isn't going to let enough light in.
^ This.
It will also be a problem with your 70-300.
If it's a traditional church building, it can get very dark, so you'll struggle to get enough light without either upping the ISO or sacrificing shutter speed (hello blurriness)
You haven't said, are you being paid for this?
If you are, then you should put some/all of the cash to either renting a good lens or buying one. Buy a decent wider prime and it will serve you well for the next event and on into the future.
Or you could buy a replacement for your kit lens, ie an 18-50mm f2.8 (constant). I have one made by Sigma, and it's great. So much better than the standard kit lens, constant f2.8 is very useful. And 18mm at the wide end is wide enough to use close up or to get groups of people in shot.
As for the party afterwards, you either need the prime because it'll be dark, or a flash.
Your 70-300 (remember that will be 105-450 on a crop sensor) will be too long for use in a party. And it certainly won't give you a wide enough aperture to be useful.
Are you going to be doing these events as the main/solo photographer? In which case you could probably do with a back up camera (borrow from a friend), spare batteries, cards, etc.
I shot a wedding last year, but only as backup to the main guy. I took my full (small) kit, Sony DSLR, 50mm f1.7 prime, 28-70mm f2.8 and 70-210mm f4. But it was outdoors, in the summer, so we had no problems with light. See some of my shots
here