Well, As per the discussion with HDB
=> You can buy a ?HDB flat while you wait for your new house, but u will not get Housing grant.
And you will have to stay in that house for 2.5yrs and should sell off when HDB gives you notice some 6months before you get the new house key.
Well, I did my research already. There are soo many houses you can buy without COV.
And there are even so many houses you can buy with less than 5k COV.
It may not be near to MRT or near to all amenities.
Well, Monthly installments will be paid from CPF. My CPF is well enough to pay the monthly installments.