The first and most important question to ask any car hire company is whether they have a car hire license or a wedding car permit.
Insurance?
What are you covered for?
Make sure you get all quotes and bookings in writing. Is there an expiry date on the quote?
Ask if you can see the cars. Is it something you want to be sitting in with your wedding dress?
What is included with the car hire. Red carpet, back tyre cover, ribbon on the car, driver?
Confirm all bookings at least 1 week prior to the wedding day. I would even go further than that a RE-CONFIRM the day before the wedding. You just never know.
Most car hire companies charge by the hour and in many cases will charge a minimum of 4 hours. Consider the time you will need the cars and how many?
Groom - home to ceremony
Bride - home to ceremony
Ceremony to photo shoot
Photo shoot to reception
Reception to first nights accommodation