it is possible to have rf signal layer on both sides (top and bottom) but for this to work properly and be manufacturable you would need a 4 layer board stackup. Top and bottom would be rf signal layers and inner layers would be ground layers. It is not very practical but it is possible.
As for the via itself you need to keep it as short as possible to avoid parasitic inductance so the thickness of the whole board needs to be minimal. Via pad size, via diameter and anti pad size will determine the impedance, which obviously needs to be 50Ohms.