my suggestion is do not give the keys until the property has been fully paid by the buyers..you are under NO obligation to give them early VP.

in the event that they start renovating, and move into the house, then out of the blue there is some snag with the the completion for whatever reason,
you will have buyers staying for free and meantime the whole process could drag on for months and months (normally its within 3 months for freehold, for leasehold around 6 months) . However, the unexpected does happen and in the end it may drag on for longer ..eg. lousy lawyers who are inefficient.

Or if I am the buyer, I tell my lawyer, err..can delay a bit ka...?  I need more time to arrange for loan lah...I don't want to start paying loan so fast lah..etc..

it will be VERY MESSY in the end esp for the seller..since you do not have a lawyer and just using the buyer's lawyer, more so you should not allow it.

An option would be for you to put in place a supplementary agreement to rent the place to them while waiting for full payment....

Generally, I would rather not as I don't want any complication with the whole process..play it safe