I'd (strongly) consider renting a lens or two:

1. Try them out and figure out what works for you.
2. You can afford to use better lenses than you can afford to buy right now.

Remember that even though you're just starting out at this, and may not have the money to buy the best lenses, the people getting married will only (we hope) be doing this once. At least in my opinion, when you're shooting a wedding, it's a moral obligation to turn down the work unless you're honestly prepared to do the job right.

Along with the lenses, I'd point to having a spare camera body as essential for a wedding. If nothing else, I'd pick up an older film body and at least a half dozen (or so) rolls of something like Vericolor (film specifically intended for portraits). Even if you're not as proficient with it, having something to ensure you don't miss truly crucial shots is a lot better than nothing (and you can pick up mid-range film bodies really inexpensively any more). Again, you might consider renting a spare body instead, at least for the first few times.