Good afternoon Eileen,
The breakdown of a private property purchase will be as such if you decide to sell off your current HDB and purchase a private property:
1) 5% cash downpayment
2) 15% CPF/ cash downpayment
3) estimated 3% buyer's stamp duty
4) misc fees like legal fees, valuation fee, stamp duty for mortgage etc.
You don't have to bear ABSD as IRAS will regard your private property purchase as your matrimonial home (provided your current is being sold).
Yes the CPF housing grant will be refunded back to your CPF OA after sale. If you do remember that the funds were being released into your OA before you guys proceeded with your HDB purchase so for sale, its the other way.
Lastly for sale, same goes for the CPF OA funds you guys have utilised on your HDB will be refunded back to OA. Sales proceeds will be generated after further deduction of any outstanding loan, CPF OA funds utilised with interest, misc fees, agent's commission etc from your sale price.
If you need any further assistance or advice for your property needs, feel free to contact me. I will asvise you with a financial plan first so from there, you guys will have a better idea to 1) sell HDB and buy private or 2) hold HDB and buy private.
Hope to hear from you soon!