Welcome to the site. Keep reading and you will find the answers to your questions and a lot you didn't know to ask.
1.Not all is adhesive but most are, heat transfer vinyl for example (for tshirts) is not.
2. All vinyl is not the same, for car windows or vehicle graphics I would suggest Oracal. 651 is good for around 5 years but a better option is 751 rated for 9 years. A lot of people use 651 for everything (myself included) but I always let the customer know its good for 5-6 years max.
3. As for the vinyl, I buy 95% of my stuff in 24" rolls. If you use a smaller roll you can adjust the pinch rollers on the plotter to adjust for the size of the roll.