HI there Harriet
don't think there is a legal minimum but the minimum amount is based upon the ongoing costs of having a SMSF.
We started with $60K - used Taxpayers Australia to help us get the documentation up and running and used our accountant who is pretty with it to keep us up to date with the ongoing requirements.
The main costs each year are the audit requirements and any subscriptions, planning costs etc to review the products being bought for the fund whether it be shares, property, antiques, paintings, other collectables etc
thanks