I am a single turning 35 soon. Will it be wise to buy a HDB flat, and pay it in full with CPF + Cash upfront?

The interest you received for your monies in CPF OA is from 2.5% to 3.5%. The interests you received from Cash holding is 0%, assuming you are not investing it. Best to minimize the use of CPF and use that as back-up plan to pay your mortgage on rainy days when you lost your income.

Just keep in mind the opportunity cost of perhaps using the money to invest elsewhere. If you’re able to get higher returns from investments than the mortgage of your HDB, it will make more sense to invest.

Also, what you’re doing is locking up your funds in buying your HDB. So make sure there are enough funds for emergencies. Although then again, it’s not too big an issue since you don’t have to worry too much about family (apart from aging parents who may need support).