If you are 100% new, try renting a couple times. Start with a short, wide beginner boat, then try again in a faster boat once you are used to the feel of a kayak.
Some shops offer demo days to try different boats.
I too paddle calm waters generally, but I do appreciate being able to get a little speed going if the mood strikes. A lot of people get the hang of the sport very quickly, so don't undershoot on the boat. Sit on tops and ultra cheap big box store boats will go slow, and only slow. People seem to love recommending terrible boats sometimes because they are "safer" (you would be surprised how many times that suggest boats that aren't all that safe) and cheap. Speed and efficiency go hand-in-hand.
You may not need a fast boat, but you will definitely appreciate efficiency. If it looks like a bathtub or something rented out at family resorts, it probably paddles as well as a barge
The car part is easy. There are kits with foam blocks and straps that work very well once you get the hang of the proper tie-down technique. Roof racks are great if you have the money, but put most of your cash towards a decent boat and a decent paddle. Other than that:
- a comfy PFD
- water shoes
- a light source
- a water bottle
- a wetsuite or waterproof jacket (depending on the where you live)
And not to forget a way to get back in your kayak when you're alone.
Take courses and learn what you are doing, kayaking is more challenging than you may think.