What to look for in a home security system?
It all depends on how much money you are willing to spend. Obvisly the contractor is pushing some ready to fit diy type security system.

In this case you need to ask him, will the unit allow you to connect to a security company such as ADT/Police via a phone line/ gprs module? as a future option. maybe get him to put a phone wire to the main box ready just to avoid redesigning later. Always good to run 1 6core and a spare 6core in case you want to add more sensors later. 
Does it have a backup power supply and how often do you have to change the battery and how much will it cost. maybe buy 1 or 2 spare in advance. Will it let you know when the batteries life time is over.(nothing lasts forever)

That was the basic part. 

Now thinking of intermidiete planning and costs at a later stage.

Does the unut generally allow you to upgrade to features such as remote access via internet (by purchasing an addon module to the main system), secure/desecure via Bluetooth, module to call another number on alarm trigger like your mobile, your partner (you set your mobile to always have a loud alarm even on silent when the alarm line calls you)

Now thinking of a more hightech and costly solution.

Will the system allow you to include cctv cameras with motion sensing and remote viewing of video via internet.These do not have to be wired- but jammers are easy to buy now a days, and cables are easy to cut--if exposed) Maybe run spare coax cables with power supply in places where you think you might want to have cameras installed and let them lead to the main box in the loft/secret space somewhere.

Ask him what is the make of the system.. and go google it. find a forum see what people have to say about- see if you can find your own prebuilt diy system that might meet your needs and has an option to expand.

If the contractor has no idea what you talking about.. get a specialist security person in to work with your building contractor.. otherwise you might regret it later.