I have heard that if you use vouchers they need to be ofsted registered
Yes, in England. If you need to part-pay using childcare vouchers, I would suggest you advertise specifically for a nanny who is already Ofsted registered, as the registration process can take some time - plus if they are already registered, you can be more confident that they meet the criteria for registration.
and also that we need to pay tax and ni.
Yes, a nanny is an employee, so you need to operate PAYE plus take into account all employment law. So things like having a written statement/contract, maternity rights, redundancy rights, holiday entitlement.
What more do I need to know or consider before I advertise?
Far too general a question in my view... probably impossible to answer as we don't know at what stage you are at.
You say you have 2 children and you are going back to work Part Time. I would wonder why you feel a nanny is your most suitable option. As cost wise it is usually more than the cost of a childminder or nursery - though does have the advantage that care is provided in your home, so no need to transport your children to someone else's home/nursery.
Read as much as you can about what a nanny does and employing a nanny. You have come to the right place, as we have loads of info on here... it can just be a little tricky to find. The childminding/nannies board has currently 343 pages of message titles, with around 50 tiles per page. So a huge number of messages there to look through.
Once you have a better understanding of what a nanny does, what rights they have, what things you need to do to be an employer... then ask us specific questions to help clarify things. We are a helpful bunch
and can usually point you in the right direction.