Will this filter be the only filter I need for a GS3 or is there something else I should look at? Total hardness of my water is 120-150.
Is there any tests I should do myself before buying a system?

Thank you for the help, everyone!

measure the general hardness and carbonate hardness. if their difference is 6° or less, use BWT Bestmax Premium, otherwise (higher difference between °GH and °KH than 6°) its recommended to install BWT Bestprotect. was told that by BWT tech in Germany... there is significant risk of gypsum formation with very unbalanced waters.