As Tazz said in his ADVICE it is best not to fit this type of alarm its like going to a shed and buying a yale alarm, but if its what you got to fit form you company then just fit it, I think they would have told the customer all about it.
As for the simcard if alarm is just made to send text all it will cost is the price of a text if on pay and go, if on a mouthy con then it will be free, if customer wants to use the app to set, unset, monitor the alarm it is better to go for a paymonthy sim or get a dedicated alarm sim.
When fitting alarm you will need full instructions which you do not get when you buy the alarm, you have to get from manufacturer website or you might be able to find a copy on the web, when fitting alarm be careful of the plastic as it is cheap and breaks easy.
If all the customer wants is a self monitored alarm and is not going to inform the insurance company that they have got one then it will do the job of letting them know that the house has been ransacked before they open the front door.
Listen to what Tazz is saying try to talk them into paying a bit more money and getting a alarm that will do the job