Don't believe an accountant that tells you that you need an accountant straight away! This is a matter of preference. There are good reasons for appointing an accountant straight away, but there will be times when it does not make sense to do so. It comes down to your preferences and objectives.
The key constraint is when is your first reporting obligation. Understand when this is (you can read a detailed treatment of this question in our Accounting Timing and Obligations post) and target appointing an accountant with enough time for them to handle this. Company formation and Payroll can be done in a matter of days. A full year of bookkeeping and accounts prep, allow a couple of months so that things are not rushed.
Subject to the above constraint, you choose when to appoint an accountant based on your requirements. The overall price that you pay does not change: