I've only done six weddings all my life, so my opinion may be limited.
If you are going to use a full frame body, it is best if you get used to all its functions and can call them up without even thinking.
A 5DII or III and a grip are recommended. More than two batteries fully charged are highly recommended.
A good quality Flash is essential
. Extra lighting and perhaps a light meter would also be very useful. Take gels too, if you know how to use them.
is absolutely VITAL
in many situations.
As for lenses, a 70-200 f/2.8 or f/4 with IS (I used 70-200mm f/2.8L IS II
for all six) is THE must have lens, along with an 85mm f/1.2 II L or 135mm f/2.0 L are the basics that most wedding photographers take with them. A 24-70 is not necessarily essential, but it is recommended.
Some photographers like to take some wide angle primes or even a 50mm lens for the "crowd" shots and the group shots, especially during the reception, and also if space is limited.
Also, have patience, since you won't really enjoy the wedding yourself
... Be ready to receive some criticism for your photographs as well. And as a rule of thumb for me, ALWAYS shoot a photo twice ... at least twice.