1. should i first find out how much loan i can get from the bank or should i find a unit which i'm interested and then ask for the bank loan
Ideally yes, please get in-principal approval from the bank for the loan amount that you are looking for. Go for a high qunatum of loan even though you take less than that.
2. do buyers normally ask for the bank's valuation of the property first before paying for the option-to-purchase
You should ask the bank to do valuation for you before giving 1%. These days it is too risky.
3. how much (rough estimation) is the miscellaneous costs (e.g. legal fees, stamp duty) that i need to take into consideration (assuming that the bank doesn't subsidize)?
Legal fee will be between 2000 to 2500. This will be subisidized by your bank. Stamp duty is around 3% plus there is 5k or so discount.
4. how long does it take, say from having the option-to-purchase, to ownership transfer of the property?
8 weeks. from the day of exercising the option.