A salesperson at a kayak shop can be very helpful, but also seek a second opinion from a kayaking friend, an instructor, Internet forums or from magazines and books. Then when you have narrowed it down to a few models, try them on the water in a safe environment, focusing on the test drive. Most shops will help you do this. Take your time and make your choice carefully. Most manufacturers supply specifications on each boat.
Some boats come with hatches, others can accommodate a later installation.
Visualize yourself using the kayak you are thinking of, like a dress rehearsal. Also consider how you will transport this kayak. Wheels that strap right to your kayak for transporting over-land come in a variety of styles. Some take apart and fit right in your hatch. As far as driving to the launch site, the size of your kayak is not relevant if you have the right roof rack system.