Why have a website ?

First and foremost, to let as many people know that the business exists and where to find you. This applies to either a business or a personal website.

Do I need to spend 100's or 1000's setting up a website ?

It all depends on what your business is and how busy you want your website. At the end of the day you are the boss and what you say goes. But generally you do not have to have an all bells and whistles website. Sometimes the simplest website can be the most effective. Having too much going on will distract the visitor and they may leave due to not know where to look first.

The website should be compatible with all devices, mobiles included.
Having clean and tidy design and most importantly factual and informative content will go a long way when it comes to getting noticed on Google and other search engines, not forgetting visitors.
Decide what information should be included and how in-depth. Ask colleagues, family and friends for their opinions to get different ideas.

Not quite, it depends on what your brand is and what you expect from your website. Having a website does not guarantee that you will be making a mint. If you have a unique brand and market it in a way that attracts visitors to either buy or pass the word about your brand then you have a fighting chance. You could open 100 shops on the same High Street but not have a single person come in; some may come in but still not buy. You need to get the word out there what street you are on and what you're selling for people to take notice; this is the whole point of a website. Once you have a website you need to tell people where to find it, the web address.

Google is one of many ways that you get the word out there what your website address is and what your brand is.
Keywords and phrases are just a few methods of telling search engines your brand and where to find you.