Just bring the handle with you when you go to a PVC suppliers as there are loads and loads of different handle sizes (from spindle to centre of cylinder).
Interlock Hardware in Deansgrange would be able to give you classy handles if you liked (Lever on Rose with Matching Escutchion) but you would have to fill in the holes which are in your door from your existing handle. The handle has to be able to lift up to lock I assume if multi point locking. You could ask for non laquered if you don't want to have the laquer pitting in a few years, but as I told you on another thread, nitromors first and then a good polish with brasso will sort a tarnish problem on brass.
The letterbox would be simple to change, again just make sure to have the measurements with you or bring it with you.
When you are going to go to the trouble of changing the other bits and bobs if it was mine (maybe you have already done so) I would change the cylinder as well as god knows who had keys while work was being done on your house.