take the door trim off and grease all the lock up and that should free it off, as for central locking they are easy to fit you can get some really nice setups depending on how much you want to spend.
most central locking kits come with instructions for wiring it up to a vac opp central locking i fitted my whole alarm system in about 4 hours but i do it for a living u should be able to do a central locking basic setup in about 1-2 hours mind you if you have no idea where to start and what wires do what and cant work it out dont guess things will go bang or just burn out
