look into www.WISPA.org and you will find the answers to your questions, and will also be able to talk with many Wireless Internet Service Providers (WISP) owners about getting started.
There is no easy answer to your questions, but looking at the questions themselves, I recommend you do a lot of research into the technology and what is available in your area. Becoming an ISP is expensive, and you have to KNOW that you will be able to get a return on investment to keep your business operational.
Survey the area to see how many customers you are likely to get, what they will pay for service, and work with hardware vendors to get bulk discounts on your antennas and equipment. You can lease internet gateway bandwidth and change it up depending on how many users you have on your network through local providers in your city or through satellite providers if you are in a rural area.
Once you get an idea of your customer base, your startup capital, your monthly expenses, and you can compare it to the amount of money + overhead expenses you will be bringing in you will have a better idea of whether this is a good idea or not.....