Why not pick something unusual? Rent a carriage, an ATV, or something.
A friend of mine actually rode to her wedding sitting in the scoop of a bulldozer that they decorated (her father works in construction, so he lent it to them). The groom showed up driving a backhoe. It was awesome, though they were kind of out towards the edge of town where you can get away with things that aren't 100% street legal.
I've seen people do horse drawn carriage, trolleys, and party buses, If you want to have a car, how about a convertable or an old car from the forties?
300 chrysler would be more stylish, but if you want something that will stand out and people will REMEMBER for ever, how about a horse and buggy with a driver in a tux????
Best wishes on your wedding.