Do you plan to sell the flat some years later?
If not, then 用綠表買未補地價的居屋 (your g/f name).
However, 用綠表買未補地價的居屋 only need to pay 5% (not 30%).
After you get marry, then you can add your name for that flat. At that moment, if your g/f like/allowded to add your name, then you can pay more $ (say 25%) into that flat.
If she is not allowded to add your name.... then you may know what kind of girl/wife she is.