it replaces the old one (mine didnt come with one) and wires into the immobiliser to bring it up to cat1. Never had a problem with it and yes, it uses all your existing stuff including the keyfob. The only annoying thing with it is that when the engines off and you want to lock the doors from the inside, you carnt without arming the alarm using the c/l button. I dont know if it does that on your existing reno one anyway - something you might want to try. Cost my £155 from an ex Renault auto electrician.
As you probably know, the reno alarm is tempremental and the toad alarm is more practical. Comes with a lifetime guarantee too but others like the clifford which is more expensive but you can add extras on apparently (not sure what) amd there is also a lazerline with deadlocks but they are over 300 quid from Eurocar.