Answer :
The south facing flat according to Vastu Shashtra you can buy if their main entrance is at suitable place . The suitable place should be as the following map :

To find the best place of main entrance you have divide the southern side of flat in 8 parts and marked 1,2,3,4,5,6,7,8. The fifth and sixth part of side is the best place of entrance . If the main entrance is not at proper place than some remedies are given below .
- Two marks of swastikas and two shape of kalash should be stick on both side of main entrance.
- Two photo of God Ganesh should be stick above the main entrance at outside and inside.