Hard to know about salary, as I'm not in Wales and I expect things could be quite different in South Wales compared with North Wales.
Try researching the local nanny market in your area by looking at job listings on NannyJob and nanny agency websites. That will then give you at least an idea.
Very ball park - anywhere between £11,000 and £22,000 Gross per year. Minimum would be National Minimum Wage (which will vary depending on the age of the worker) though I expect around £11,000 Gross per year.
E-Gismos Calculator for PAYE will give you a good indication of likely Employers NI figure.
On a salary of £11,000 it's roughly £700. With £22,000 salary, it's roughly £2100.
What you can negotiate in terms of wage will vary depending on the candidate. You may find that you can find someone who only wants to work Term Time, if that suits you.
No idea about the Registration system in Wales. Childminders can register with someone - CSIW - but I don't know about nannies.
Duties will vary... generally it is things to do with the children, so cooking for the children, children's washing & putting away clothes. Taking children to/from school, activities, toddler group, etc. You will find quite a bit of discussion on this board if you look through old threads with regard to duties nannies do and also other interesting topics, such as typical weekly kitty money, info about operating PAYE, drawing up a nanny contract, problems that occur etc.