PCX 150cc when new can reach 120kmh, older bike will probably lose some speed. I don't think it's enough for relaxing highway riding. 100kmh to 120kmh will be at a snails pace so probably only over taking in the emergency lane.

IMO for scooter 250cc is a minimum for highway duty with average top speed around 130kmh to 140 kmh. Enough power to overtake at a safe pace. 250cc are also heavier and wider so less wind buffeting from high speed and when passing lorries. Major service for scooter can be around RM500-600 for replacing all the fluids, oil brake, shocks coolant. All filters and new pads plus new spark plugs and cvt belting.