If you want just central locking then look on ebay, remote central locking kits around £30, don't bother with the ford stuff, it's not as good ime, would be easier fit but can be unreliable, I use after market every time for cl kits.
If wanting to use with a alarm, then you don't need the remote function of the CL kit, so instead go to maplins, about £20 for a 4 door kit, I fitted one of these about 5 years ago on the astra for the gf and it's still perfect 5 years on.
To this I added a boot/hatch motor from the scrappies and simply joined into one side.
It's an absolute doddle to wire up, and to link in to the alarm.
For the alarm if wanted I would go toad, if you want a approved one a ai606, would need fitting and certing by someone, but if not fussed a toad sterling is the same quality, just research a bit to see what features you want in the different ones.
tabetha