Should we clean out savings for EC?

A $900K property will require at least $180K downpayment. If the purchase will wipe out both your CPF and savings, it is definitely not prudent. It is likely that as you are self employed, a 30% haircut for the purposes of your loan might be imposed by the bank if they consider your income variable.

Let's do some simple maths for the worst case scenario. Your income would be considered $10.15K, and applying the mortgage servicing ratio of 30% means your repayments are capped at $3045 per month. A loan of $720K over 30 years with an interest of 2.2% leads to a $2734 monthly payment. There's only a small buffer and not forgetting the other costs that come with a new flat purchase which you must consider, such as legal fees, etc. Also you must remember that you will mortgage insurance on your property and there are probably other fees to be considered as well.

In short, it's not a wise idea. You will be left with no emergency savings, which is an extremely dangerous move. Even if you could secure money from his dad, both of you will have sleepless nights over this. Your emergency savings must also grow to include a buffer for a year's worth of repayments should you go ahead to purchase. (As you are both self employed without much CPF, so presumbly you will be paying cash. Hence the emergency fund needs to be bigger) If business conditions affect your income, you need to ask yourself what is the backup plan to repay the loan.

Not recommended to wipe out your savings/cpf for property.

Do keep an emergency fund (~6 month expenses) in case of sudden changes to your income or other emergencies so that you are still able to pay for the mortgage every month.