Find a local company that is Nacoss approved and that you like the look of. I found a Nacoss gold installer and they fitted me a Visonic Powermax system after I was broken into a few month ago. It was cheaper than the equivalent ADT system and probably better quality!
Nacoss Silver can be just as good tbh, Gold just have to adhere to more standards. But some smaller companies don't have the time/resources for all the paperwork.
TBH ADT don't really want Joe Bloggs, they want national accounts. OP go for a local company. It was probably over quoted as the salesman won't see a large commision.
I'd also recommend any of the following manufacturers:
Honeywell (pref Galaxy, not the FLEX panel, as it is utter pap)
Risco/Gardtec (not the Prosys panel) (PIR's, DT's, door contacts etc from these are probably the best I've ever used)
Menvier
Scantronic
Texecom (British made, the Premier panel is very good, Veritas is ok but doesn't come with all the bells & whistles)
Wiring can (depends on the installer, be laid under carpet at the sides, next to gripper rods)
I'd try and stay away from wireless systems, they really should be used as a get out of jail card. However if you do have to go wireless. It would have to be a Gardtec system imo.
Any questions just ask.
Edit: ohh and if you need some kind of monitoring (ie: police response) I recommend CSL Dualcom. We have a regular on these forums that works for CSL in their technical support department.