To beceom a marketer, you need exactly zero qualifications. A 10 year old waltzing into the company could theoretically be a marketer, if the legal hiring age wasn't mandatory 16 (in practice this is unlikely, because like a lot of jobs that involves judgement and skill, you need experience). As far as I know, this is true for UK, USA, Canada, Australia, and New Zealand. I have not checked the requirements in other countries.
The CIM is though a good qualification to have. For one, you can use it practically anywhere in the world. Second, the relevant levels would make it the equivalent of specific degrees, where you get the qualification at a fraction of the cost and completed in a fraction of the time e.g. less than 1 year to complete.
In terms of pay, as far as I know marketing is about what results you can get, and your level of skills and what sort of experience you have (if you can bring in a lot of business, you will be paid accordingly for example). This is why they don't really care about what qualifications you have and pretty much anyone can join. Whether you thrive is another matter.
Having said that, if you know next to nothing about marketing, CIM is a good way to say that you at least know the basics. Whether you want to go for the higher levels after you complete level 3 is up to you.
Personal bias: I prefer CIM over other UK professional marketing qualifications e.g. IDM, IPA.