If I like to select and buy a flat already completed construction, is Sale of Balance Flats (SBF) the correct place to go to?

SBF will allow you to shorten the waiting time for your flat (since most SBF have already begun construction or on rare occasion, even completed construction, as opposed to BTOs), and will be cheaper than resales (although still more expensive than BTOs).

Do note that often SBFs are quite limited in quantity and the location offerings so you might not be able to get a unit that you want. Moreover, if I'm not mistaken, SBF offerings only occur twice per year so it might be a long wait if the locations you want are not offered.

Good luck!