The first is to be fairly conservative with your choice of beer. The first time I ordered a keg, it was Anchor Steam and almost no one drank it.
The last time I ordered kegs for an event, it was two sixtels (1/6 barrel) instead of full kegs (1/2 bbl). One of them was hard cider and the other Sam Adams Oktoberfest, both which were very popular. We actually ran out too quickly, though because it was a low budget wedding, we couldn't really have bought much more.
Serving beer is more complicated than most people think. I went to one wedding on a very, very warm day (one of the bridesmaids passed out from the heat) where the caterers were pouring mostly foam and had no idea what to do about it.
My serving recommendation to you is to borrow/rent a jockey box from a homebrew store or distributor. These are coolers that have either coils or a plate chiller which takes room temp beer down to 34 degrees in seconds. You can leave the kegs under a table & run the hoses up the back. The front of the cooler you can decorate & frilly it up for the wedding.
I highly recommend using a jockey box & it shouldn't cost too much to rent.
Another great benefit of using a jockey box is that the beer is usually pushed with CO2. This is way better than the hand pumps that most frat parties use. The hand pumps put oxygen on the beer and it will go stale very fast. If you push the beer with CO2 it will stay fresh much longer (more or less indefinitely).
Jockey boxes only foam if there is no ice under the plate chiller or if there are bubbles in the line.
So if your wedding doesn't drink all of the beer you can finish it at a later date without throwing it all away.
If you have long beer lines and don't cool them, you get foam. If it's a hot day and you don't keep the kegs properly cooled, you get foam. If you shake the kegs too much in transit, you get foam. Experienced caterers should know what they need to do to get good pours.
Keep in mind that beer begins spoiling when it comes into contact with air.
That means if you use standard hand-pumped taps, you have at most a couple days to consume everything in the kegs. If you find a way to use CO2, not only will the beer still be good to drink for weeks afterward (if refrigerated properly), the pours will be faster and more consistent. If you can rent a "jockey box", you could theoretically hide the kegs and only have the faucets be visible. (I've never used one, but the idea is you're cooling the beer as it comes through the lines rather than the whole keg.)
The big caveat with not using standard hand-pumped taps is that foreign beers use different style keg couplers, so you would have to make sure to get the correct one if you were buying a Belgian, for example.
If you can't rent a jockey box from somewhere, check with local breweries or homebrew clubs. A lot of breweries and homebrewers have jockey boxes available for brew events and competitions, etc. They might be willing to let you borrow it for the weekend for free, or for a couple bucks. Just a thought. Also check craigslist, I see them on there from time to time.