If I use UV polyester resin, do I need to buy MEKP catalisyst or only the wax surfacing (hot coat) and styren (gloss coat)?

Yes, you need to add MEKP catalyst to UV cure resin for finbox and leash plug installs because the sun's UV rays will not penetrate that deep into the board. 

It is also a good idea to add a drop of catalyst to UV resin for laminating and hotcoating so any resin that hits the floor will cure over time if not exposed to the sun.

Also, if using resin pigments you need to add MEKP catalyst for full cure...