The problem is for £400 you will not get very good kit, and poor kit normally goes wrong. This means as said before you will be returning all the time trying to fix stuff you supplied under warranty. You obviously are not making any profit on the kit itself, so get him to buy the kit and you fit it, that way if the kit goes wrong you can harge him to come and take the bit down so he can take it back to the supplier and change it, then you can charge him for refitting it. If you supply the kit and sell it for the same price you paid for it all you are doing is making yourself busy for no money. Either that or you wont return his calls when it goes wrong and he wont b recommending you to anyone. Personally I would walk away his budget is not enough to achieve a reasonable system. Cheap CCTV is like filling your car up with water instead of petrol, it gives the impression that you have a full tank, until you try driving somewhere.
Where are you? Everyone I know is busier than a Asian kiddy in a Nike factory