Remote central locking kits easier to install if central locking already in place, as just extra module to wire in. Plenty of advice on here re wiring, and careful use of a circuit tester helps!
Also plenty of complete kits to convert non-central locking cars to remote central locking; would be more involved as motors/actuators need fitting to the cars, etc.
But as said, time, patience and a circuit tester work well. Failing that, local auto-electrician shouldnt charge a fortune.