Welcome!
I prefer a local bike shop, one where you are treated well and seriously, to buy a bike. Sometimes you have to shop around and talk to several people to find the shop that fits you. Getting a front wheel off is quite easy with the quick release lever. You could also get a hitch rack, among other options.
As far as hard bodies on bikes-they prolly didn't start out that way, just as you aren't. People of all shapes, ages and sizes fit on two wheels just fine and I've found form is little indicator of function. What is it they say? You have to love yourself before others can love you? If your body image is anything like mine you are WAY too hard on yourself. Be proud of yourself for taking steps in improving your own health and make sure you notice ALL the people you see on a bike, you'll see what I mean. When I see other people I just think, cool, another bike rider! and wave or nod.
Join a local ride for people of all abilities, it's a great way to learn and to enjoy time on a bike. You'll need a helmet and I highly recommend learning how to fix flat tires-it may seem daunting but is actually pretty simple once you get the hang of it. It's certainly much better than walking! You'll be shifting gears without giving it much thought after lots of practice. Ride and ride and ride and before too long you'll be able to help the new riders out with your own tips!
Just remember, everyone had to start somewhere. I can't think of a single person who jumped on the bike and became superbiker instantly. Welcome to the board, I can't wait to see what great information the girls have for you!
I used to have an open mind but my brains kept falling out.