I own an Initiale which has the Carminat system as standard. I suspect that updating a car not so equipped would be a real headache – the system uses a CD player situated in the glove box , which receives Sat Nav signals from the shark fin on the roof – you may need to plum in a suitable aerial if yours does not have all the wiring present. Then you need the control head unit, and also the screen, which may require a dash/clocks change, as the non sat nav models don't have a large screen fitted.
After doing all that you would end up with a not very good system which cannot be updated (latest disc made was 2014/15), so in a word I would say don't bother! Even if you could get all the kit for free I am not sure whether you may have to change parts of the wiring loom between the various units, and I would say it is never worth the hassle when compared to buying a Tom Tom or similar system.
As regards your query about the handbrake I can't help I am afraid. My own hill holder can sometimes not appear to work – certainly unless you have come to a halt and applied the brakes hard it doesn't work, and can slip back sometimes on steep slopes. I have very occasionally used my left foot on the brake while applying throttle with the right to get me going….