Checkout a Surly Crosscheck if you can. http://surlybikes.com/bikes/cross_check. You will get used to dismounting as your flexibility will increase the more you ride. At first I had to lean the bike way over to dismount. I remember almost busting my azz while dismounting during a test ride. I find it helpful to bend my leg at the knee as I'm swinging my leg over the seat. You'll get the hang of it quickly.

Different shops will offer different levels of fitting services. Some might just have Uncle Buck come out and visually size you up, while others will offer a professional fitting service that uses a video camera to record your peddling and position. Whatever method, it's critical you find a bike that fits and is comfortable to you. Don't hesitate to test ride bikes.