Dielectric and surface finish

[joniengr081]

Hi, I have read that "The speed of the propagating EM wave is inversely proportional to the dielectric constant. Hence, a low Dk PCB material will have the following advantages: High-speed signals propagate with lower losses. Reduced crosstalk between closely spaced traces and vias."
Which dielectric should be used in the application include high speed ADC design up to 2.5 GSPS connected to ZYNQ Ultrascale+ on a custom develop board having 2 GByte LPDDR4 RAM. The custom board also contains peripherals like 10GigE, USB 3.0, PCIe, and SD-Card. This is a complex deign. That's why I am wondering if FR4 is still an option or a stackup with lower dielectric constant material should be considered ?

What is the difference between these surface finishes: ENIG｜HAL｜HAL lead free｜OSP｜Immersion Sn｜Immersion silver｜Plating hard gold｜Plating silver