You can get software to help do the payroll. Payroll providers don't just help by doing payslips, they also assist with contract and are there for general enquiries about anything nanny related. You could try doing it yourself at first, agree a gross salary, but do make sure you understand how to do it.
Pension wise, as a new employer you won't need to provide a pension scheme until Jan 2018. You will need to appoint a contact, which if you are doing payroll yourself would be you.
We use Paye for nannies and have used nanny paye. Both great. It costs very little (just over £100) so yes I would recommend. Just making all the payments is a hassle enough this makes everything else straight forward. Especially useful For SSP, calculating pay when nanny leaves etc.