My bunch are certainly very well used to travelling! They've collectively prob got more miles than Jeremy Clarkson
Litter: I always have a litter tray in the carrier with them as usually the shortest journey I am doing with mine is 7-8 hours. Sticky back velcro on the bottom of the litter tray and the bottom of the cage will ensure it stays in place rather than flying round the carrier and hitting your cat. As an alternative to litter tray you could line the bottom of the carrier with puppy training pads although I've found these slide around too much and end up bunched up in a corner. Just because they have a litter tray in the carrier doesn't always mean they'll use it. Boopy prefers to cross her legs and wait until she gets to wherever she is going or until we stop overnight at a hotel.
Food: differs from animal to animal, if you feed Bilbo on a break in a journey he'll throw up as soon as you start moving again but can quite happily go 9 hours without food whereas Boopy can eat whilst the car is moving with no problems at all. I never withold food before a journey.
Water: I usually just give them a bowl of water each time I stop and leave the car door open whilst I have a drink and bite to eat in the car so they can get some fresh air. If your cat can fit their head in one of the little anti-spill bowls that fits onto the door of the carrier then it would be a good idea to get one of those so they can drink on the go if they want to as well.
Stress: I'm very lucky that mine just settle down and go to sleep in the carrier unless both Dave and I are in the car and then Boopy squeals as she knows I worry and knows that I will sit in the back with her and she can curl up and go to sleep on my lap. Might sounds cruel but I've found if you ignore the squealing they soon stop. Might eb an idea to try Feliway spray although I have never found it to make any difference.
Also start leaving the carrier out in your house and putting treats/toys/food in it so that your cat associates it with nice things rather than horrible things like the vets as that will help with their stress levels as well.
I always make sure I've got baby wipes, kitchen roll, nappy bags, spare bedding/blankets, big bottle of water, some wet food and bowls in their travel bag.