Robin,
Difficult to answer as you don't mention which school or what your housing budget is. My family and I have just relocated as of December so I can offer you this.
Housing deposits are normally 1 month's rent for a one year lease and 2 months for two years. Not sure if this is law or just common practice but it was what I was told. Depending on the agent you use, you may also end up paying 1/2 to 1 full months rent as an agent fee. If you use the agents associated with this site, rents over S$2500 per month are not charged an agents fee.
As for schooling, the norm we found was 1 term/semester tuition was required as a deposit. They also try to get you to pay the entire year and deposit upfront but if needed, I'm sure you could work something out with the school. I found most of the
International Schools to be from S$7000 - S$12,000 per semester/term.
You will also need to consider some other smaller deposits such as utilities, cable TV etc. We recently set up our utilities and because it is a private contract (not through our company) and I am on an EP, the deposit was S$500. Cable TV was S$250.
So depending on what you are looking for in terms of housing, what school you are considering etc 30K may or may not be enough for your particular situation. Hope this helps.