An easy and convenient way to go about would be -

a] for PCR: you can isolate the DNA and store it accordingly. DNA is a stable molecule and you can use it whenever you want.

b]for cytometry: you can fix the cells and store them easily. You can go through the normal steps of preparing cells for cytometry and just stop before the staining step, Suspend them in the wash buffer you use (the step of washing after permeabilization ) and store them at -80 until further use, which hopefully would not be after a long gap.

Appreciated. But is it gonna be a direct way of preserving and storing buffy coat it self ?

In my opinion you can just fix the cells and keep them in your -80. You just want to store them and not use them for other live cell experiments, so in that case that should suffice.